diff --git a/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/RepositoryTestCase.java b/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/RepositoryTestCase.java
index 95fe18b..987f923 100644
--- a/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/RepositoryTestCase.java
+++ b/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/RepositoryTestCase.java
@@ -349,7 +349,8 @@
 	 * younger modification timestamp than the modification timestamp of the
 	 * given file. This is done by touching a temporary file, reading the
 	 * lastmodified attribute and, if needed, sleeping. After sleeping this loop
-	 * starts again until the filesystem timer has advanced enough.
+	 * starts again until the filesystem timer has advanced enough. The
+	 * temporary file will be created as a sibling of lastFile.
 	 *
 	 * @param lastFile
 	 *            the file on which we want to wait until the filesystem timer
@@ -362,21 +363,26 @@
 	 */
 	public static long fsTick(File lastFile) throws InterruptedException,
 			IOException {
-		long sleepTime = 64;
+		File tmp;
 		FS fs = FS.DETECTED;
-		if (lastFile != null && !fs.exists(lastFile))
-			throw new FileNotFoundException(lastFile.getPath());
-		File tmp = File.createTempFile("FileTreeIteratorWithTimeControl", null);
+		if (lastFile == null) {
+			lastFile = tmp = File
+					.createTempFile("fsTickTmpFile", null);
+		} else {
+			if (!fs.exists(lastFile)) {
+				throw new FileNotFoundException(lastFile.getPath());
+			}
+			tmp = File.createTempFile("fsTickTmpFile", null,
+					lastFile.getParentFile());
+		}
+		long res = FS.getFsTimerResolution(tmp.toPath()).toMillis();
+		long sleepTime = res / 10;
 		try {
-			long startTime = (lastFile == null) ? fs.lastModified(tmp) : fs
-					.lastModified(lastFile);
+			long startTime = fs.lastModified(lastFile);
 			long actTime = fs.lastModified(tmp);
 			while (actTime <= startTime) {
 				Thread.sleep(sleepTime);
-				sleepTime *= 2;
-				try (FileOutputStream fos = new FileOutputStream(tmp)) {
-					// Do nothing
-				}
+				FileUtils.touch(tmp.toPath());
 				actTime = fs.lastModified(tmp);
 			}
 			return actTime;
diff --git a/org.eclipse.jgit.lfs.server.test/BUILD b/org.eclipse.jgit.lfs.server.test/BUILD
index 1341dd6..fb0d691 100644
--- a/org.eclipse.jgit.lfs.server.test/BUILD
+++ b/org.eclipse.jgit.lfs.server.test/BUILD
@@ -32,7 +32,7 @@
         exclude = TEST_BASE,
     ),
     jvm_flags = [
-        "-Xmx256m",
+        "-Xmx512m",
         "-Dfile.encoding=UTF-8",
     ],
     tags = ["lfs-server"],
diff --git a/org.eclipse.jgit.pgm.test/BUILD b/org.eclipse.jgit.pgm.test/BUILD
index 5bedf9a..f32fa12 100644
--- a/org.eclipse.jgit.pgm.test/BUILD
+++ b/org.eclipse.jgit.pgm.test/BUILD
@@ -7,7 +7,7 @@
     name = "pgm",
     srcs = glob(["tst/**/*.java"]),
     jvm_flags = [
-        "-Xmx256m",
+        "-Xmx512m",
         "-Dfile.encoding=UTF-8",
     ],
     tags = ["pgm"],
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/FileSnapshotTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/FileSnapshotTest.java
index 9ceaa34..5ebdeb6 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/FileSnapshotTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/FileSnapshotTest.java
@@ -42,16 +42,22 @@
  */
 package org.eclipse.jgit.internal.storage.file;
 
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
 import java.io.File;
 import java.io.FileOutputStream;
 import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.StandardCopyOption;
+import java.nio.file.attribute.FileTime;
 import java.util.ArrayList;
 import java.util.List;
 
 import org.eclipse.jgit.util.FileUtils;
+import org.eclipse.jgit.util.SystemReader;
 import org.junit.After;
+import org.junit.Assume;
 import org.junit.Before;
 import org.junit.Test;
 
@@ -121,12 +127,59 @@
 	@Test
 	public void testNewFileNoWait() throws Exception {
 		File f1 = createFile("newfile");
-		waitNextSec(f1);
 		FileSnapshot save = FileSnapshot.save(f1);
-		Thread.sleep(1500);
 		assertTrue(save.isModified(f1));
 	}
 
+	/**
+	 * Simulate packfile replacement in same file which may occur if set of
+	 * objects in the pack is the same but pack config was different. On Posix
+	 * filesystems this should change the inode (filekey in java.nio
+	 * terminology).
+	 *
+	 * @throws Exception
+	 */
+	@Test
+	public void testSimulatePackfileReplacement() throws Exception {
+		Assume.assumeFalse(SystemReader.getInstance().isWindows());
+		File f1 = createFile("file"); // inode y
+		File f2 = createFile("fool"); // Guarantees new inode x
+		// wait on f2 since this method resets lastModified of the file
+		// and leaves lastModified of f1 untouched
+		waitNextSec(f2);
+		waitNextSec(f2);
+		FileTime timestamp = Files.getLastModifiedTime(f1.toPath());
+		FileSnapshot save = FileSnapshot.save(f1);
+		Files.move(f2.toPath(), f1.toPath(), // Now "file" is inode x
+				StandardCopyOption.REPLACE_EXISTING,
+				StandardCopyOption.ATOMIC_MOVE);
+		Files.setLastModifiedTime(f1.toPath(), timestamp);
+		assertTrue(save.isModified(f1));
+		assertTrue("unexpected change of fileKey", save.wasFileKeyChanged());
+		assertFalse("unexpected size change", save.wasSizeChanged());
+		assertFalse("unexpected lastModified change",
+				save.wasLastModifiedChanged());
+		assertFalse("lastModified was unexpectedly racily clean",
+				save.wasLastModifiedRacilyClean());
+	}
+
+	/**
+	 * Append a character to a file to change its size and set original
+	 * lastModified
+	 *
+	 * @throws Exception
+	 */
+	@Test
+	public void testFileSizeChanged() throws Exception {
+		File f = createFile("file");
+		FileTime timestamp = Files.getLastModifiedTime(f.toPath());
+		FileSnapshot save = FileSnapshot.save(f);
+		append(f, (byte) 'x');
+		Files.setLastModifiedTime(f.toPath(), timestamp);
+		assertTrue(save.isModified(f));
+		assertTrue(save.wasSizeChanged());
+	}
+
 	private File createFile(String string) throws IOException {
 		trash.mkdirs();
 		File f = File.createTempFile(string, "tdat", trash);
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/PackFileSnapshotTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/PackFileSnapshotTest.java
new file mode 100644
index 0000000..a1433e9
--- /dev/null
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/PackFileSnapshotTest.java
@@ -0,0 +1,354 @@
+/*
+ * Copyright (C) 2019, Matthias Sohn <matthias.sohn@sap.com>
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package org.eclipse.jgit.internal.storage.file;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assume.assumeFalse;
+import static org.junit.Assume.assumeTrue;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.io.Writer;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.nio.file.StandardCopyOption;
+import java.nio.file.StandardOpenOption;
+//import java.nio.file.attribute.BasicFileAttributes;
+import java.text.ParseException;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.Random;
+import java.util.zip.Deflater;
+
+import org.eclipse.jgit.api.GarbageCollectCommand;
+import org.eclipse.jgit.api.Git;
+import org.eclipse.jgit.api.errors.AbortedByHookException;
+import org.eclipse.jgit.api.errors.ConcurrentRefUpdateException;
+import org.eclipse.jgit.api.errors.GitAPIException;
+import org.eclipse.jgit.api.errors.NoFilepatternException;
+import org.eclipse.jgit.api.errors.NoHeadException;
+import org.eclipse.jgit.api.errors.NoMessageException;
+import org.eclipse.jgit.api.errors.UnmergedPathsException;
+import org.eclipse.jgit.api.errors.WrongRepositoryStateException;
+import org.eclipse.jgit.junit.RepositoryTestCase;
+import org.eclipse.jgit.lib.AnyObjectId;
+import org.eclipse.jgit.lib.ConfigConstants;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.storage.file.FileBasedConfig;
+import org.eclipse.jgit.storage.pack.PackConfig;
+import org.junit.Test;
+
+public class PackFileSnapshotTest extends RepositoryTestCase {
+
+	private static ObjectId unknownID = ObjectId
+			.fromString("1234567890123456789012345678901234567890");
+
+	@Test
+	public void testSamePackDifferentCompressionDetectChecksumChanged()
+			throws Exception {
+		Git git = Git.wrap(db);
+		File f = writeTrashFile("file", "foobar ");
+		for (int i = 0; i < 10; i++) {
+			appendRandomLine(f);
+			git.add().addFilepattern("file").call();
+			git.commit().setMessage("message" + i).call();
+		}
+
+		FileBasedConfig c = db.getConfig();
+		c.setInt(ConfigConstants.CONFIG_GC_SECTION, null,
+				ConfigConstants.CONFIG_KEY_AUTOPACKLIMIT, 1);
+		c.save();
+		Collection<PackFile> packs = gc(Deflater.NO_COMPRESSION);
+		assertEquals("expected 1 packfile after gc", 1, packs.size());
+		PackFile p1 = packs.iterator().next();
+		PackFileSnapshot snapshot = p1.getFileSnapshot();
+
+		packs = gc(Deflater.BEST_COMPRESSION);
+		assertEquals("expected 1 packfile after gc", 1, packs.size());
+		PackFile p2 = packs.iterator().next();
+		File pf = p2.getPackFile();
+
+		// changing compression level with aggressive gc may change size,
+		// fileKey (on *nix) and checksum. Hence FileSnapshot.isModified can
+		// return true already based on size or fileKey.
+		// So the only thing we can test here is that we ensure that checksum
+		// also changed when we read it here in this test
+		assertTrue("expected snapshot to detect modified pack",
+				snapshot.isModified(pf));
+		assertTrue("expected checksum changed", snapshot.isChecksumChanged(pf));
+	}
+
+	private void appendRandomLine(File f, int length, Random r)
+			throws IOException {
+		try (Writer w = Files.newBufferedWriter(f.toPath(),
+				StandardOpenOption.APPEND)) {
+			appendRandomLine(w, length, r);
+		}
+	}
+
+	private void appendRandomLine(File f) throws IOException {
+		appendRandomLine(f, 5, new Random());
+	}
+
+	private void appendRandomLine(Writer w, int len, Random r)
+			throws IOException {
+		final int c1 = 32; // ' '
+		int c2 = 126; // '~'
+		for (int i = 0; i < len; i++) {
+			w.append((char) (c1 + r.nextInt(1 + c2 - c1)));
+		}
+	}
+
+	private ObjectId createTestRepo(int testDataSeed, int testDataLength)
+			throws IOException, GitAPIException, NoFilepatternException,
+			NoHeadException, NoMessageException, UnmergedPathsException,
+			ConcurrentRefUpdateException, WrongRepositoryStateException,
+			AbortedByHookException {
+		// Create a repo with two commits and one file. Each commit adds
+		// testDataLength number of bytes. Data are random bytes. Since the
+		// seed for the random number generator is specified we will get
+		// the same set of bytes for every run and for every platform
+		Random r = new Random(testDataSeed);
+		Git git = Git.wrap(db);
+		File f = writeTrashFile("file", "foobar ");
+		appendRandomLine(f, testDataLength, r);
+		git.add().addFilepattern("file").call();
+		git.commit().setMessage("message1").call();
+		appendRandomLine(f, testDataLength, r);
+		git.add().addFilepattern("file").call();
+		return git.commit().setMessage("message2").call().getId();
+	}
+
+	// Try repacking so fast that you get two new packs which differ only in
+	// content/chksum but have same name, size and lastmodified.
+	// Since this is done with standard gc (which creates new tmp files and
+	// renames them) the filekeys of the new packfiles differ helping jgit
+	// to detect the fast modification
+	@Test
+	public void testDetectModificationAlthoughSameSizeAndModificationtime()
+			throws Exception {
+		int testDataSeed = 1;
+		int testDataLength = 100;
+		FileBasedConfig config = db.getConfig();
+		// don't use mtime of the parent folder to detect pack file
+		// modification.
+		config.setBoolean(ConfigConstants.CONFIG_CORE_SECTION, null,
+				ConfigConstants.CONFIG_KEY_TRUSTFOLDERSTAT, false);
+		config.save();
+
+		createTestRepo(testDataSeed, testDataLength);
+
+		// repack to create initial packfile
+		PackFile pf = repackAndCheck(5, null, null, null);
+		Path packFilePath = pf.getPackFile().toPath();
+		AnyObjectId chk1 = pf.getPackChecksum();
+		String name = pf.getPackName();
+		Long length = Long.valueOf(pf.getPackFile().length());
+		long m1 = packFilePath.toFile().lastModified();
+
+		// Wait for a filesystem timer tick to enhance probability the rest of
+		// this test is done before the filesystem timer ticks again.
+		fsTick(packFilePath.toFile());
+
+		// Repack to create packfile with same name, length. Lastmodified and
+		// content and checksum are different since compression level differs
+		AnyObjectId chk2 = repackAndCheck(6, name, length, chk1)
+				.getPackChecksum();
+		long m2 = packFilePath.toFile().lastModified();
+		assumeFalse(m2 == m1);
+
+		// Repack to create packfile with same name, length. Lastmodified is
+		// equal to the previous one because we are in the same filesystem timer
+		// slot. Content and its checksum are different
+		AnyObjectId chk3 = repackAndCheck(7, name, length, chk2)
+				.getPackChecksum();
+		long m3 = packFilePath.toFile().lastModified();
+
+		// ask for an unknown git object to force jgit to rescan the list of
+		// available packs. If we would ask for a known objectid then JGit would
+		// skip searching for new/modified packfiles
+		db.getObjectDatabase().has(unknownID);
+		assertEquals(chk3, getSinglePack(db.getObjectDatabase().getPacks())
+				.getPackChecksum());
+		assumeTrue(m3 == m2);
+	}
+
+	// Try repacking so fast that we get two new packs which differ only in
+	// content and checksum but have same name, size and lastmodified.
+	// To avoid that JGit detects modification by checking the filekey create
+	// two new packfiles upfront and create copies of them. Then modify the
+	// packfiles in-place by opening them for write and then copying the
+	// content.
+	@Test
+	public void testDetectModificationAlthoughSameSizeAndModificationtimeAndFileKey()
+			throws Exception {
+		int testDataSeed = 1;
+		int testDataLength = 100;
+		FileBasedConfig config = db.getConfig();
+		config.setBoolean(ConfigConstants.CONFIG_CORE_SECTION, null,
+				ConfigConstants.CONFIG_KEY_TRUSTFOLDERSTAT, false);
+		config.save();
+
+		createTestRepo(testDataSeed, testDataLength);
+
+		// Repack to create initial packfile. Make a copy of it
+		PackFile pf = repackAndCheck(5, null, null, null);
+		Path packFilePath = pf.getPackFile().toPath();
+		Path packFileBasePath = packFilePath.resolveSibling(
+				packFilePath.getFileName().toString().replaceAll(".pack", ""));
+		AnyObjectId chk1 = pf.getPackChecksum();
+		String name = pf.getPackName();
+		Long length = Long.valueOf(pf.getPackFile().length());
+		copyPack(packFileBasePath, "", ".copy1");
+
+		// Repack to create second packfile. Make a copy of it
+		AnyObjectId chk2 = repackAndCheck(6, name, length, chk1)
+				.getPackChecksum();
+		copyPack(packFileBasePath, "", ".copy2");
+
+		// Repack to create third packfile
+		AnyObjectId chk3 = repackAndCheck(7, name, length, chk2)
+				.getPackChecksum();
+		long m3 = packFilePath.toFile().lastModified();
+		db.getObjectDatabase().has(unknownID);
+		assertEquals(chk3, getSinglePack(db.getObjectDatabase().getPacks())
+				.getPackChecksum());
+
+		// Wait for a filesystem timer tick to enhance probability the rest of
+		// this test is done before the filesystem timer ticks.
+		fsTick(packFilePath.toFile());
+
+		// Copy copy2 to packfile data to force modification of packfile without
+		// changing the packfile's filekey.
+		copyPack(packFileBasePath, ".copy2", "");
+		long m2 = packFilePath.toFile().lastModified();
+		assumeFalse(m3 == m2);
+
+		db.getObjectDatabase().has(unknownID);
+		assertEquals(chk2, getSinglePack(db.getObjectDatabase().getPacks())
+				.getPackChecksum());
+
+		// Copy copy2 to packfile data to force modification of packfile without
+		// changing the packfile's filekey.
+		copyPack(packFileBasePath, ".copy1", "");
+		long m1 = packFilePath.toFile().lastModified();
+		assumeTrue(m2 == m1);
+		db.getObjectDatabase().has(unknownID);
+		assertEquals(chk1, getSinglePack(db.getObjectDatabase().getPacks())
+				.getPackChecksum());
+	}
+
+	// Copy file from src to dst but avoid creating a new File (with new
+	// FileKey) if dst already exists
+	private Path copyFile(Path src, Path dst) throws IOException {
+		if (Files.exists(dst)) {
+			dst.toFile().setWritable(true);
+			try (OutputStream dstOut = Files.newOutputStream(dst)) {
+				Files.copy(src, dstOut);
+				return dst;
+			}
+		} else {
+			return Files.copy(src, dst, StandardCopyOption.REPLACE_EXISTING);
+		}
+	}
+
+	private Path copyPack(Path base, String srcSuffix, String dstSuffix)
+			throws IOException {
+		copyFile(Paths.get(base + ".idx" + srcSuffix),
+				Paths.get(base + ".idx" + dstSuffix));
+		copyFile(Paths.get(base + ".bitmap" + srcSuffix),
+				Paths.get(base + ".bitmap" + dstSuffix));
+		return copyFile(Paths.get(base + ".pack" + srcSuffix),
+				Paths.get(base + ".pack" + dstSuffix));
+	}
+
+	private PackFile repackAndCheck(int compressionLevel, String oldName,
+			Long oldLength, AnyObjectId oldChkSum)
+			throws IOException, ParseException {
+		PackFile p = getSinglePack(gc(compressionLevel));
+		File pf = p.getPackFile();
+		// The following two assumptions should not cause the test to fail. If
+		// on a certain platform we get packfiles (containing the same git
+		// objects) where the lengths differ or the checksums don't differ we
+		// just skip this test. A reason for that could be that compression
+		// works differently or random number generator works differently. Then
+		// we have to search for more consistent test data or checkin these
+		// packfiles as test resources
+		assumeTrue(oldLength == null || pf.length() == oldLength.longValue());
+		assumeTrue(oldChkSum == null || !p.getPackChecksum().equals(oldChkSum));
+		assertTrue(oldName == null || p.getPackName().equals(oldName));
+		return p;
+	}
+
+	private PackFile getSinglePack(Collection<PackFile> packs) {
+		Iterator<PackFile> pIt = packs.iterator();
+		PackFile p = pIt.next();
+		assertFalse(pIt.hasNext());
+		return p;
+	}
+
+	private Collection<PackFile> gc(int compressionLevel)
+			throws IOException, ParseException {
+		GC gc = new GC(db);
+		PackConfig pc = new PackConfig(db.getConfig());
+		pc.setCompressionLevel(compressionLevel);
+
+		pc.setSinglePack(true);
+
+		// --aggressive
+		pc.setDeltaSearchWindowSize(
+				GarbageCollectCommand.DEFAULT_GC_AGGRESSIVE_WINDOW);
+		pc.setMaxDeltaDepth(GarbageCollectCommand.DEFAULT_GC_AGGRESSIVE_DEPTH);
+		pc.setReuseObjects(false);
+
+		gc.setPackConfig(pc);
+		gc.setExpireAgeMillis(0);
+		gc.setPackExpireAgeMillis(0);
+		return gc.gc();
+	}
+
+}
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/FSTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/FSTest.java
index 5293ca4..6dfa6ef 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/FSTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/FSTest.java
@@ -46,17 +46,24 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
-import static org.junit.Assume.assumeTrue;
 import static org.junit.Assume.assumeNoException;
+import static org.junit.Assume.assumeTrue;
 
 import java.io.File;
 import java.io.IOException;
 import java.nio.charset.Charset;
 import java.nio.file.Files;
 import java.nio.file.InvalidPathException;
+import java.nio.file.Path;
+import java.nio.file.attribute.FileTime;
 import java.nio.file.attribute.PosixFileAttributeView;
 import java.nio.file.attribute.PosixFilePermission;
+import java.time.Duration;
+import java.time.ZoneId;
+import java.time.format.DateTimeFormatter;
+import java.util.Locale;
 import java.util.Set;
+import java.util.concurrent.TimeUnit;
 
 import org.eclipse.jgit.errors.CommandFailedException;
 import org.eclipse.jgit.junit.RepositoryTestCase;
@@ -215,4 +222,34 @@
 				  new String[] { "this-command-does-not-exist" },
 				  Charset.defaultCharset().name());
 	}
+
+	@Test
+	public void testFsTimestampResolution() throws Exception {
+		DateTimeFormatter formatter = DateTimeFormatter
+				.ofPattern("uuuu-MMM-dd HH:mm:ss.nnnnnnnnn", Locale.ENGLISH)
+				.withZone(ZoneId.systemDefault());
+		Path dir = Files.createTempDirectory("probe-filesystem");
+		Duration resolution = FS.getFsTimerResolution(dir);
+		long resolutionNs = resolution.toNanos();
+		assertTrue(resolutionNs > 0);
+		for (int i = 0; i < 10; i++) {
+			Path f = null;
+			try {
+				f = dir.resolve("testTimestampResolution" + i);
+				Files.createFile(f);
+				FileUtils.touch(f);
+				FileTime t1 = Files.getLastModifiedTime(f);
+				TimeUnit.NANOSECONDS.sleep(resolutionNs);
+				FileUtils.touch(f);
+				FileTime t2 = Files.getLastModifiedTime(f);
+				assertTrue(String.format(
+						"expected t2=%s to be larger than t1=%s\nsince file timestamp resolution was measured to be %,d ns",
+						formatter.format(t2.toInstant()),
+						formatter.format(t1.toInstant()),
+						Long.valueOf(resolutionNs)), t2.compareTo(t1) > 0);
+			} finally {
+				Files.delete(f);
+			}
+		}
+	}
 }
diff --git a/org.eclipse.jgit/.settings/.api_filters b/org.eclipse.jgit/.settings/.api_filters
index 01ceac0..eb93d3e 100644
--- a/org.eclipse.jgit/.settings/.api_filters
+++ b/org.eclipse.jgit/.settings/.api_filters
@@ -22,4 +22,76 @@
             </message_arguments>
         </filter>
     </resource>
+    <resource path="src/org/eclipse/jgit/storage/pack/PackConfig.java" type="org.eclipse.jgit.storage.pack.PackConfig">
+        <filter id="336658481">
+            <message_arguments>
+                <message_argument value="org.eclipse.jgit.storage.pack.PackConfig"/>
+                <message_argument value="DEFAULT_MINSIZE_PREVENT_RACY_PACK"/>
+            </message_arguments>
+        </filter>
+        <filter id="336658481">
+            <message_arguments>
+                <message_argument value="org.eclipse.jgit.storage.pack.PackConfig"/>
+                <message_argument value="DEFAULT_WAIT_PREVENT_RACY_PACK"/>
+            </message_arguments>
+        </filter>
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.1.8"/>
+                <message_argument value="DEFAULT_MINSIZE_PREVENT_RACY_PACK"/>
+            </message_arguments>
+        </filter>
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.1.8"/>
+                <message_argument value="DEFAULT_WAIT_PREVENT_RACY_PACK"/>
+            </message_arguments>
+        </filter>
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.1.8"/>
+                <message_argument value="doWaitPreventRacyPack(long)"/>
+            </message_arguments>
+        </filter>
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.1.8"/>
+                <message_argument value="getMinSizePreventRacyPack()"/>
+            </message_arguments>
+        </filter>
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.1.8"/>
+                <message_argument value="isWaitPreventRacyPack()"/>
+            </message_arguments>
+        </filter>
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.1.8"/>
+                <message_argument value="setMinSizePreventRacyPack(long)"/>
+            </message_arguments>
+        </filter>
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.1.8"/>
+                <message_argument value="setWaitPreventRacyPack(boolean)"/>
+            </message_arguments>
+        </filter>
+    </resource>
+    <resource path="src/org/eclipse/jgit/util/FS.java" type="org.eclipse.jgit.util.FS">
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.2.3"/>
+                <message_argument value="getFsTimerResolution(Path)"/>
+            </message_arguments>
+        </filter>
+    </resource>
+    <resource path="src/org/eclipse/jgit/util/FileUtils.java" type="org.eclipse.jgit.util.FileUtils">
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.2.3"/>
+                <message_argument value="touch(Path)"/>
+            </message_arguments>
+        </filter>
+    </resource>
 </component>
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileSnapshot.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileSnapshot.java
index f26eba3..1de3135 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileSnapshot.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileSnapshot.java
@@ -48,9 +48,13 @@
 import java.nio.file.attribute.BasicFileAttributes;
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
+import java.time.Duration;
 import java.util.Date;
 import java.util.Locale;
+import java.util.Objects;
+import java.util.concurrent.TimeUnit;
 
+import org.eclipse.jgit.annotations.NonNull;
 import org.eclipse.jgit.util.FS;
 
 /**
@@ -77,6 +81,8 @@
 	 */
 	public static final long UNKNOWN_SIZE = -1;
 
+	private static final Object MISSING_FILEKEY = new Object();
+
 	/**
 	 * A FileSnapshot that is considered to always be modified.
 	 * <p>
@@ -84,7 +90,8 @@
 	 * file, but only after {@link #isModified(File)} gets invoked. The returned
 	 * snapshot contains only invalid status information.
 	 */
-	public static final FileSnapshot DIRTY = new FileSnapshot(-1, -1, UNKNOWN_SIZE);
+	public static final FileSnapshot DIRTY = new FileSnapshot(-1, -1,
+			UNKNOWN_SIZE, Duration.ZERO, MISSING_FILEKEY);
 
 	/**
 	 * A FileSnapshot that is clean if the file does not exist.
@@ -93,7 +100,8 @@
 	 * file to be clean. {@link #isModified(File)} will return false if the file
 	 * path does not exist.
 	 */
-	public static final FileSnapshot MISSING_FILE = new FileSnapshot(0, 0, 0) {
+	public static final FileSnapshot MISSING_FILE = new FileSnapshot(0, 0, 0,
+			Duration.ZERO, MISSING_FILEKEY) {
 		@Override
 		public boolean isModified(File path) {
 			return FS.DETECTED.exists(path);
@@ -111,18 +119,12 @@
 	 * @return the snapshot.
 	 */
 	public static FileSnapshot save(File path) {
-		long read = System.currentTimeMillis();
-		long modified;
-		long size;
-		try {
-			BasicFileAttributes fileAttributes = FS.DETECTED.fileAttributes(path);
-			modified = fileAttributes.lastModifiedTime().toMillis();
-			size = fileAttributes.size();
-		} catch (IOException e) {
-			modified = path.lastModified();
-			size = path.length();
-		}
-		return new FileSnapshot(read, modified, size);
+		return new FileSnapshot(path);
+	}
+
+	private static Object getFileKey(BasicFileAttributes fileAttributes) {
+		Object fileKey = fileAttributes.fileKey();
+		return fileKey == null ? MISSING_FILEKEY : fileKey;
 	}
 
 	/**
@@ -130,6 +132,11 @@
 	 * already known.
 	 * <p>
 	 * This method should be invoked before the file is accessed.
+	 * <p>
+	 * Note that this method cannot rely on measuring file timestamp resolution
+	 * to avoid racy git issues caused by finite file timestamp resolution since
+	 * it's unknown in which filesystem the file is located. Hence the worst
+	 * case fallback for timestamp resolution is used.
 	 *
 	 * @param modified
 	 *            the last modification time of the file
@@ -137,7 +144,8 @@
 	 */
 	public static FileSnapshot save(long modified) {
 		final long read = System.currentTimeMillis();
-		return new FileSnapshot(read, modified, -1);
+		return new FileSnapshot(read, modified, -1, Duration.ZERO,
+				MISSING_FILEKEY);
 	}
 
 	/** Last observed modification time of the path. */
@@ -154,11 +162,57 @@
 	 * When set to {@link #UNKNOWN_SIZE} the size is not considered for modification checks. */
 	private final long size;
 
-	private FileSnapshot(long read, long modified, long size) {
+	/** measured filesystem timestamp resolution */
+	private Duration fsTimestampResolution;
+
+	/**
+	 * Object that uniquely identifies the given file, or {@code
+	 * null} if a file key is not available
+	 */
+	private final Object fileKey;
+
+	/**
+	 * Record a snapshot for a specific file path.
+	 * <p>
+	 * This method should be invoked before the file is accessed.
+	 *
+	 * @param path
+	 *            the path to later remember. The path's current status
+	 *            information is saved.
+	 */
+	protected FileSnapshot(File path) {
+		this.lastRead = System.currentTimeMillis();
+		this.fsTimestampResolution = FS
+				.getFsTimerResolution(path.toPath().getParent());
+		BasicFileAttributes fileAttributes = null;
+		try {
+			fileAttributes = FS.DETECTED.fileAttributes(path);
+		} catch (IOException e) {
+			this.lastModified = path.lastModified();
+			this.size = path.length();
+			this.fileKey = MISSING_FILEKEY;
+			return;
+		}
+		this.lastModified = fileAttributes.lastModifiedTime().toMillis();
+		this.size = fileAttributes.size();
+		this.fileKey = getFileKey(fileAttributes);
+	}
+
+	private boolean sizeChanged;
+
+	private boolean fileKeyChanged;
+
+	private boolean lastModifiedChanged;
+
+	private boolean wasRacyClean;
+
+	private FileSnapshot(long read, long modified, long size,
+			@NonNull Duration fsTimestampResolution, @NonNull Object fileKey) {
 		this.lastRead = read;
 		this.lastModified = modified;
-		this.cannotBeRacilyClean = notRacyClean(read);
+		this.fsTimestampResolution = fsTimestampResolution;
 		this.size = size;
+		this.fileKey = fileKey;
 	}
 
 	/**
@@ -187,15 +241,30 @@
 	public boolean isModified(File path) {
 		long currLastModified;
 		long currSize;
+		Object currFileKey;
 		try {
 			BasicFileAttributes fileAttributes = FS.DETECTED.fileAttributes(path);
 			currLastModified = fileAttributes.lastModifiedTime().toMillis();
 			currSize = fileAttributes.size();
+			currFileKey = getFileKey(fileAttributes);
 		} catch (IOException e) {
 			currLastModified = path.lastModified();
 			currSize = path.length();
+			currFileKey = MISSING_FILEKEY;
 		}
-		return (currSize != UNKNOWN_SIZE && currSize != size) || isModified(currLastModified);
+		sizeChanged = isSizeChanged(currSize);
+		if (sizeChanged) {
+			return true;
+		}
+		fileKeyChanged = isFileKeyChanged(currFileKey);
+		if (fileKeyChanged) {
+			return true;
+		}
+		lastModifiedChanged = isModified(currLastModified);
+		if (lastModifiedChanged) {
+			return true;
+		}
+		return false;
 	}
 
 	/**
@@ -222,12 +291,26 @@
 	 */
 	public void setClean(FileSnapshot other) {
 		final long now = other.lastRead;
-		if (notRacyClean(now))
+		if (!isRacyClean(now)) {
 			cannotBeRacilyClean = true;
+		}
 		lastRead = now;
 	}
 
 	/**
+	 * Wait until this snapshot's file can't be racy anymore
+	 *
+	 * @throws InterruptedException
+	 *             if sleep was interrupted
+	 */
+	public void waitUntilNotRacy() throws InterruptedException {
+		while (isRacyClean(System.currentTimeMillis())) {
+			TimeUnit.NANOSECONDS
+					.sleep((fsTimestampResolution.toNanos() + 1) * 11 / 10);
+		}
+	}
+
+	/**
 	 * Compare two snapshots to see if they cache the same information.
 	 *
 	 * @param other
@@ -235,72 +318,120 @@
 	 * @return true if the two snapshots share the same information.
 	 */
 	public boolean equals(FileSnapshot other) {
-		return lastModified == other.lastModified;
+		return lastModified == other.lastModified && size == other.size
+				&& Objects.equals(fileKey, other.fileKey);
 	}
 
 	/** {@inheritDoc} */
 	@Override
-	public boolean equals(Object other) {
-		if (other instanceof FileSnapshot)
-			return equals((FileSnapshot) other);
-		return false;
+	public boolean equals(Object obj) {
+		if (this == obj) {
+			return true;
+		}
+		if (obj == null) {
+			return false;
+		}
+		if (!(obj instanceof FileSnapshot)) {
+			return false;
+		}
+		FileSnapshot other = (FileSnapshot) obj;
+		return equals(other);
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public int hashCode() {
-		// This is pretty pointless, but override hashCode to ensure that
-		// x.hashCode() == y.hashCode() when x.equals(y) is true.
-		//
-		return (int) lastModified;
+		return Objects.hash(Long.valueOf(lastModified), Long.valueOf(size),
+				fileKey);
+	}
+
+	/**
+	 * @return {@code true} if FileSnapshot.isModified(File) found the file size
+	 *         changed
+	 */
+	boolean wasSizeChanged() {
+		return sizeChanged;
+	}
+
+	/**
+	 * @return {@code true} if FileSnapshot.isModified(File) found the file key
+	 *         changed
+	 */
+	boolean wasFileKeyChanged() {
+		return fileKeyChanged;
+	}
+
+	/**
+	 * @return {@code true} if FileSnapshot.isModified(File) found the file's
+	 *         lastModified changed
+	 */
+	boolean wasLastModifiedChanged() {
+		return lastModifiedChanged;
+	}
+
+	/**
+	 * @return {@code true} if FileSnapshot.isModified(File) detected that
+	 *         lastModified is racily clean
+	 */
+	boolean wasLastModifiedRacilyClean() {
+		return wasRacyClean;
 	}
 
 	/** {@inheritDoc} */
+	@SuppressWarnings("nls")
 	@Override
 	public String toString() {
-		if (this == DIRTY)
-			return "DIRTY"; //$NON-NLS-1$
-		if (this == MISSING_FILE)
-			return "MISSING_FILE"; //$NON-NLS-1$
-		DateFormat f = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS", //$NON-NLS-1$
+		if (this == DIRTY) {
+			return "DIRTY";
+		}
+		if (this == MISSING_FILE) {
+			return "MISSING_FILE";
+		}
+		DateFormat f = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS",
 				Locale.US);
-		return "FileSnapshot[modified: " + f.format(new Date(lastModified)) //$NON-NLS-1$
-				+ ", read: " + f.format(new Date(lastRead)) + "]"; //$NON-NLS-1$ //$NON-NLS-2$
+		return "FileSnapshot[modified: " + f.format(new Date(lastModified))
+				+ ", read: " + f.format(new Date(lastRead)) + ", size:" + size
+				+ ", fileKey: " + fileKey + "]";
 	}
 
-	private boolean notRacyClean(long read) {
-		// The last modified time granularity of FAT filesystems is 2 seconds.
-		// Using 2.5 seconds here provides a reasonably high assurance that
-		// a modification was not missed.
-		//
-		return read - lastModified > 2500;
+	private boolean isRacyClean(long read) {
+		// add a 10% safety margin
+		long racyNanos = (fsTimestampResolution.toNanos() + 1) * 11 / 10;
+		return wasRacyClean = (read - lastModified) * 1_000_000 <= racyNanos;
 	}
 
 	private boolean isModified(long currLastModified) {
 		// Any difference indicates the path was modified.
-		//
-		if (lastModified != currLastModified)
+
+		lastModifiedChanged = lastModified != currLastModified;
+		if (lastModifiedChanged) {
 			return true;
+		}
 
 		// We have already determined the last read was far enough
 		// after the last modification that any new modifications
 		// are certain to change the last modified time.
-		//
-		if (cannotBeRacilyClean)
+		if (cannotBeRacilyClean) {
 			return false;
-
-		if (notRacyClean(lastRead)) {
+		}
+		if (!isRacyClean(lastRead)) {
 			// Our last read should have marked cannotBeRacilyClean,
 			// but this thread may not have seen the change. The read
 			// of the volatile field lastRead should have fixed that.
-			//
 			return false;
 		}
 
 		// We last read this path too close to its last observed
 		// modification time. We may have missed a modification.
 		// Scan again, to ensure we still see the same state.
-		//
 		return true;
 	}
+
+	private boolean isFileKeyChanged(Object currFileKey) {
+		return currFileKey != MISSING_FILEKEY && !currFileKey.equals(fileKey);
+	}
+
+	private boolean isSizeChanged(long currSize) {
+		return currSize != UNKNOWN_SIZE && currSize != size;
+	}
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/GC.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/GC.java
index a791ac7..00124bc 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/GC.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/GC.java
@@ -177,7 +177,7 @@
 
 	private Date packExpire;
 
-	private PackConfig pconfig = null;
+	private PackConfig pconfig;
 
 	/**
 	 * the refs which existed during the last call to {@link #repack()}. This is
@@ -213,6 +213,7 @@
 	 */
 	public GC(FileRepository repo) {
 		this.repo = repo;
+		this.pconfig = new PackConfig(repo);
 		this.pm = NullProgressMonitor.INSTANCE;
 	}
 
@@ -397,7 +398,7 @@
 	 */
 	private void removeOldPack(File packFile, String packName, PackExt ext,
 			int deleteOptions) throws IOException {
-		if (pconfig != null && pconfig.isPreserveOldPacks()) {
+		if (pconfig.isPreserveOldPacks()) {
 			File oldPackDir = repo.getObjectDatabase().getPreservedDirectory();
 			FileUtils.mkdir(oldPackDir, true);
 
@@ -413,7 +414,7 @@
 	 * Delete the preserved directory including all pack files within
 	 */
 	private void prunePreserved() {
-		if (pconfig != null && pconfig.isPrunePreserved()) {
+		if (pconfig.isPrunePreserved()) {
 			try {
 				FileUtils.delete(repo.getObjectDatabase().getPreservedDirectory(),
 						FileUtils.RECURSIVE | FileUtils.RETRY | FileUtils.SKIP_MISSING);
@@ -855,7 +856,7 @@
 		nonHeads.addAll(indexObjects);
 
 		// Combine the GC_REST objects into the GC pack if requested
-		if (pconfig != null && pconfig.getSinglePack()) {
+		if (pconfig.getSinglePack()) {
 			allHeadsAndTags.addAll(nonHeads);
 			nonHeads.clear();
 		}
@@ -1159,7 +1160,7 @@
 			return Integer.signum(o1.hashCode() - o2.hashCode());
 		});
 		try (PackWriter pw = new PackWriter(
-				(pconfig == null) ? new PackConfig(repo) : pconfig,
+				pconfig,
 				repo.newObjectReader())) {
 			// prepare the PackWriter
 			pw.setDeltaBaseAsOffset(true);
@@ -1255,8 +1256,23 @@
 							realExt), e);
 				}
 			}
-
-			return repo.getObjectDatabase().openPack(realPack);
+			boolean interrupted = false;
+			try {
+				FileSnapshot snapshot = FileSnapshot.save(realPack);
+				if (pconfig.doWaitPreventRacyPack(snapshot.size())) {
+					snapshot.waitUntilNotRacy();
+				}
+			} catch (InterruptedException e) {
+				interrupted = true;
+			}
+			try {
+				return repo.getObjectDatabase().openPack(realPack);
+			} finally {
+				if (interrupted) {
+					// Re-set interrupted flag
+					Thread.currentThread().interrupt();
+				}
+			}
 		} finally {
 			if (tmpPack != null && tmpPack.exists())
 				tmpPack.delete();
@@ -1434,7 +1450,7 @@
 	 *            the {@link org.eclipse.jgit.storage.pack.PackConfig} used when
 	 *            writing packs
 	 */
-	public void setPackConfig(PackConfig pconfig) {
+	public void setPackConfig(@NonNull PackConfig pconfig) {
 		this.pconfig = pconfig;
 	}
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectory.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectory.java
index d502fea..258ccee 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectory.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectory.java
@@ -910,9 +910,10 @@
 
 			final String packName = base + PACK.getExtension();
 			final File packFile = new File(packDirectory, packName);
-			final PackFile oldPack = forReuse.remove(packName);
+			final PackFile oldPack = forReuse.get(packName);
 			if (oldPack != null
 					&& !oldPack.getFileSnapshot().isModified(packFile)) {
+				forReuse.remove(packName);
 				list.add(oldPack);
 				continue;
 			}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectoryPackParser.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectoryPackParser.java
index 0cec2d5..6e8a15e 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectoryPackParser.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectoryPackParser.java
@@ -65,6 +65,7 @@
 import org.eclipse.jgit.lib.CoreConfig;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ProgressMonitor;
+import org.eclipse.jgit.storage.pack.PackConfig;
 import org.eclipse.jgit.transport.PackParser;
 import org.eclipse.jgit.transport.PackedObjectInfo;
 import org.eclipse.jgit.util.FileUtils;
@@ -122,9 +123,12 @@
 	/** The pack that was created, if parsing was successful. */
 	private PackFile newPack;
 
+	private PackConfig pconfig;
+
 	ObjectDirectoryPackParser(FileObjectDatabase odb, InputStream src) {
 		super(odb, src);
 		this.db = odb;
+		this.pconfig = new PackConfig(odb.getConfig());
 		this.crc = new CRC32();
 		this.tailDigest = Constants.newMessageDigest();
 
@@ -514,6 +518,15 @@
 					JGitText.get().cannotMoveIndexTo, finalIdx), e);
 		}
 
+		boolean interrupted = false;
+		try {
+			FileSnapshot snapshot = FileSnapshot.save(finalPack);
+			if (pconfig.doWaitPreventRacyPack(snapshot.size())) {
+				snapshot.waitUntilNotRacy();
+			}
+		} catch (InterruptedException e) {
+			interrupted = true;
+		}
 		try {
 			newPack = db.openPack(finalPack);
 		} catch (IOException err) {
@@ -523,6 +536,11 @@
 			if (finalIdx.exists())
 				FileUtils.delete(finalIdx);
 			throw err;
+		} finally {
+			if (interrupted) {
+				// Re-set interrupted flag
+				Thread.currentThread().interrupt();
+			}
 		}
 
 		return lockMessage != null ? keep : null;
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackFile.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackFile.java
index d834b1a..73ad38c 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackFile.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackFile.java
@@ -93,6 +93,8 @@
 import org.eclipse.jgit.util.LongList;
 import org.eclipse.jgit.util.NB;
 import org.eclipse.jgit.util.RawParseUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * A Git version 2 pack file representation. A pack file contains Git objects in
@@ -100,6 +102,7 @@
  * objects are similar.
  */
 public class PackFile implements Iterable<PackIndex.MutableEntry> {
+	private final static Logger LOG = LoggerFactory.getLogger(PackFile.class);
 	/** Sorts PackFiles to be most recently created to least recently created. */
 	public static final Comparator<PackFile> SORT = new Comparator<PackFile>() {
 		@Override
@@ -131,7 +134,7 @@
 
 	int packLastModified;
 
-	private FileSnapshot fileSnapshot;
+	private PackFileSnapshot fileSnapshot;
 
 	private volatile boolean invalid;
 
@@ -168,7 +171,7 @@
 	 */
 	public PackFile(File packFile, int extensions) {
 		this.packFile = packFile;
-		this.fileSnapshot = FileSnapshot.save(packFile);
+		this.fileSnapshot = PackFileSnapshot.save(packFile);
 		this.packLastModified = (int) (fileSnapshot.lastModified() >> 10);
 		this.extensions = extensions;
 
@@ -189,10 +192,22 @@
 						throw new PackInvalidException(packFile, invalidatingCause);
 					}
 					try {
+						long start = System.currentTimeMillis();
 						idx = PackIndex.open(extFile(INDEX));
+						if (LOG.isDebugEnabled()) {
+							LOG.debug(String.format(
+									"Opening pack index %s, size %.3f MB took %d ms", //$NON-NLS-1$
+									extFile(INDEX).getAbsolutePath(),
+									Float.valueOf(extFile(INDEX).length()
+											/ (1024f * 1024)),
+									Long.valueOf(System.currentTimeMillis()
+											- start)));
+						}
 
 						if (packChecksum == null) {
 							packChecksum = idx.packChecksum;
+							fileSnapshot.setChecksum(
+									ObjectId.fromRaw(packChecksum));
 						} else if (!Arrays.equals(packChecksum,
 								idx.packChecksum)) {
 							throw new PackMismatchException(MessageFormat
@@ -371,10 +386,14 @@
 	 *
 	 * @return the packfile @{@link FileSnapshot} that the object is loaded from.
 	 */
-	FileSnapshot getFileSnapshot() {
+	PackFileSnapshot getFileSnapshot() {
 		return fileSnapshot;
 	}
 
+	AnyObjectId getPackChecksum() {
+		return ObjectId.fromRaw(packChecksum);
+	}
+
 	private final byte[] decompress(final long position, final int sz,
 			final WindowCursor curs) throws IOException, DataFormatException {
 		byte[] dstbuf;
@@ -1209,4 +1228,12 @@
 	private boolean hasExt(PackExt ext) {
 		return (extensions & ext.getBit()) != 0;
 	}
+
+	@SuppressWarnings("nls")
+	@Override
+	public String toString() {
+		return "PackFile [packFileName=" + packFile.getName() + ", length="
+				+ packFile.length() + ", packChecksum="
+				+ ObjectId.fromRaw(packChecksum).name() + "]";
+	}
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackFileSnapshot.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackFileSnapshot.java
new file mode 100644
index 0000000..19ec3af
--- /dev/null
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackFileSnapshot.java
@@ -0,0 +1,123 @@
+/*
+ * Copyright (C) 2019, Matthias Sohn <matthias.sohn@sap.com>
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package org.eclipse.jgit.internal.storage.file;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.RandomAccessFile;
+
+import org.eclipse.jgit.lib.AnyObjectId;
+import org.eclipse.jgit.lib.ObjectId;
+
+class PackFileSnapshot extends FileSnapshot {
+
+	private static final ObjectId MISSING_CHECKSUM = ObjectId.zeroId();
+
+	/**
+	 * Record a snapshot for a specific packfile path.
+	 * <p>
+	 * This method should be invoked before the packfile is accessed.
+	 *
+	 * @param path
+	 *            the path to later remember. The path's current status
+	 *            information is saved.
+	 * @return the snapshot.
+	 */
+	public static PackFileSnapshot save(File path) {
+		return new PackFileSnapshot(path);
+	}
+
+	private AnyObjectId checksum = MISSING_CHECKSUM;
+
+	private boolean wasChecksumChanged;
+
+
+	PackFileSnapshot(File packFile) {
+		super(packFile);
+	}
+
+	void setChecksum(AnyObjectId checksum) {
+		this.checksum = checksum;
+	}
+
+	/** {@inheritDoc} */
+	@Override
+	public boolean isModified(File packFile) {
+		if (!super.isModified(packFile)) {
+			return false;
+		}
+		if (wasSizeChanged() || wasFileKeyChanged()
+				|| !wasLastModifiedRacilyClean()) {
+			return true;
+		}
+		return isChecksumChanged(packFile);
+	}
+
+	boolean isChecksumChanged(File packFile) {
+		return wasChecksumChanged = checksum != MISSING_CHECKSUM
+				&& !checksum.equals(readChecksum(packFile));
+	}
+
+	private AnyObjectId readChecksum(File packFile) {
+		try (RandomAccessFile fd = new RandomAccessFile(packFile, "r")) { //$NON-NLS-1$
+			fd.seek(fd.length() - 20);
+			final byte[] buf = new byte[20];
+			fd.readFully(buf, 0, 20);
+			return ObjectId.fromRaw(buf);
+		} catch (IOException e) {
+			return MISSING_CHECKSUM;
+		}
+	}
+
+	boolean wasChecksumChanged() {
+		return wasChecksumChanged;
+	}
+
+	@SuppressWarnings("nls")
+	@Override
+	public String toString() {
+		return "PackFileSnapshot [checksum=" + checksum + ", "
+				+ super.toString() + "]";
+	}
+
+}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackInserter.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackInserter.java
index 0ce3cc9..a27a2b0 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackInserter.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackInserter.java
@@ -86,6 +86,7 @@
 import org.eclipse.jgit.lib.ObjectLoader;
 import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.ObjectStream;
+import org.eclipse.jgit.storage.pack.PackConfig;
 import org.eclipse.jgit.transport.PackParser;
 import org.eclipse.jgit.transport.PackedObjectInfo;
 import org.eclipse.jgit.util.BlockList;
@@ -115,8 +116,11 @@
 	private PackStream packOut;
 	private Inflater cachedInflater;
 
+	private PackConfig pconfig;
+
 	PackInserter(ObjectDirectory db) {
 		this.db = db;
+		this.pconfig = new PackConfig(db.getConfig());
 	}
 
 	/**
@@ -296,9 +300,25 @@
 					realIdx), e);
 		}
 
-		db.openPack(realPack);
-		rollback = false;
-		clear();
+		boolean interrupted = false;
+		try {
+			FileSnapshot snapshot = FileSnapshot.save(realPack);
+			if (pconfig.doWaitPreventRacyPack(snapshot.size())) {
+				snapshot.waitUntilNotRacy();
+			}
+		} catch (InterruptedException e) {
+			interrupted = true;
+		}
+		try {
+			db.openPack(realPack);
+			rollback = false;
+		} finally {
+			clear();
+			if (interrupted) {
+				// Re-set interrupted flag
+				Thread.currentThread().interrupt();
+			}
+		}
 	}
 
 	private static void writePackIndex(File idx, byte[] packHash,
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/PackConfig.java b/org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/PackConfig.java
index ed3fe2a..6722e9b 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/PackConfig.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/PackConfig.java
@@ -116,12 +116,30 @@
 	 */
 	public static final int DEFAULT_DELTA_SEARCH_WINDOW_SIZE = 10;
 
+	private static final int MB = 1 << 20;
+
 	/**
 	 * Default big file threshold: {@value}
 	 *
 	 * @see #setBigFileThreshold(int)
 	 */
-	public static final int DEFAULT_BIG_FILE_THRESHOLD = 50 * 1024 * 1024;
+	public static final int DEFAULT_BIG_FILE_THRESHOLD = 50 * MB;
+
+	/**
+	 * Default if we wait before opening a newly written pack to prevent its
+	 * lastModified timestamp could be racy
+	 *
+	 * @since 5.1.8
+	 */
+	public static final boolean DEFAULT_WAIT_PREVENT_RACY_PACK = false;
+
+	/**
+	 * Default if we wait before opening a newly written pack to prevent its
+	 * lastModified timestamp could be racy
+	 *
+	 * @since 5.1.8
+	 */
+	public static final long DEFAULT_MINSIZE_PREVENT_RACY_PACK = 100 * MB;
 
 	/**
 	 * Default delta cache size: {@value}
@@ -238,6 +256,10 @@
 
 	private int bigFileThreshold = DEFAULT_BIG_FILE_THRESHOLD;
 
+	private boolean waitPreventRacyPack = DEFAULT_WAIT_PREVENT_RACY_PACK;
+
+	private long minSizePreventRacyPack = DEFAULT_MINSIZE_PREVENT_RACY_PACK;
+
 	private int threads;
 
 	private Executor executor;
@@ -314,6 +336,8 @@
 		this.deltaCacheSize = cfg.deltaCacheSize;
 		this.deltaCacheLimit = cfg.deltaCacheLimit;
 		this.bigFileThreshold = cfg.bigFileThreshold;
+		this.waitPreventRacyPack = cfg.waitPreventRacyPack;
+		this.minSizePreventRacyPack = cfg.minSizePreventRacyPack;
 		this.threads = cfg.threads;
 		this.executor = cfg.executor;
 		this.indexVersion = cfg.indexVersion;
@@ -737,6 +761,76 @@
 	}
 
 	/**
+	 * Get whether we wait before opening a newly written pack to prevent its
+	 * lastModified timestamp could be racy
+	 *
+	 * @return whether we wait before opening a newly written pack to prevent
+	 *         its lastModified timestamp could be racy
+	 * @since 5.1.8
+	 */
+	public boolean isWaitPreventRacyPack() {
+		return waitPreventRacyPack;
+	}
+
+	/**
+	 * Get whether we wait before opening a newly written pack to prevent its
+	 * lastModified timestamp could be racy. Returns {@code true} if
+	 * {@code waitToPreventRacyPack = true} and
+	 * {@code packSize > minSizePreventRacyPack}, {@code false} otherwise.
+	 *
+	 * @param packSize
+	 *            size of the pack file
+	 *
+	 * @return whether we wait before opening a newly written pack to prevent
+	 *         its lastModified timestamp could be racy
+	 * @since 5.1.8
+	 */
+	public boolean doWaitPreventRacyPack(long packSize) {
+		return isWaitPreventRacyPack()
+				&& packSize > getMinSizePreventRacyPack();
+	}
+
+	/**
+	 * Set whether we wait before opening a newly written pack to prevent its
+	 * lastModified timestamp could be racy
+	 *
+	 * @param waitPreventRacyPack
+	 *            whether we wait before opening a newly written pack to prevent
+	 *            its lastModified timestamp could be racy
+	 * @since 5.1.8
+	 */
+	public void setWaitPreventRacyPack(boolean waitPreventRacyPack) {
+		this.waitPreventRacyPack = waitPreventRacyPack;
+	}
+
+	/**
+	 * Get minimum packfile size for which we wait before opening a newly
+	 * written pack to prevent its lastModified timestamp could be racy if
+	 * {@code isWaitToPreventRacyPack} is {@code true}.
+	 *
+	 * @return minimum packfile size, default is 100 MiB
+	 *
+	 * @since 5.1.8
+	 */
+	public long getMinSizePreventRacyPack() {
+		return minSizePreventRacyPack;
+	}
+
+	/**
+	 * Set minimum packfile size for which we wait before opening a newly
+	 * written pack to prevent its lastModified timestamp could be racy if
+	 * {@code isWaitToPreventRacyPack} is {@code true}.
+	 *
+	 * @param minSizePreventRacyPack
+	 *            minimum packfile size, default is 100 MiB
+	 *
+	 * @since 5.1.8
+	 */
+	public void setMinSizePreventRacyPack(long minSizePreventRacyPack) {
+		this.minSizePreventRacyPack = minSizePreventRacyPack;
+	}
+
+	/**
 	 * Get the compression level applied to objects in the pack.
 	 *
 	 * Default setting: {@value java.util.zip.Deflater#DEFAULT_COMPRESSION}
@@ -1083,6 +1177,10 @@
 		setBitmapInactiveBranchAgeInDays(
 				rc.getInt("pack", "bitmapinactivebranchageindays", //$NON-NLS-1$ //$NON-NLS-2$
 						getBitmapInactiveBranchAgeInDays()));
+		setWaitPreventRacyPack(rc.getBoolean("pack", "waitpreventracypack", //$NON-NLS-1$ //$NON-NLS-2$
+				isWaitPreventRacyPack()));
+		setMinSizePreventRacyPack(rc.getLong("pack", "minsizepreventracypack", //$NON-NLS-1$//$NON-NLS-2$
+				getMinSizePreventRacyPack()));
 	}
 
 	/** {@inheritDoc} */
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java
index 536e4f1..fb95887 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java
@@ -55,23 +55,31 @@
 import java.io.OutputStream;
 import java.io.PrintStream;
 import java.nio.charset.Charset;
+import java.nio.file.AccessDeniedException;
+import java.nio.file.FileStore;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.attribute.BasicFileAttributes;
+import java.nio.file.attribute.FileTime;
 import java.security.AccessController;
 import java.security.PrivilegedAction;
 import java.text.MessageFormat;
+import java.time.Duration;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Collectors;
 
+import org.eclipse.jgit.annotations.NonNull;
 import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.api.errors.JGitInternalException;
 import org.eclipse.jgit.errors.CommandFailedException;
@@ -180,6 +188,83 @@
 		}
 	}
 
+	private static final class FileStoreAttributeCache {
+		/**
+		 * The last modified time granularity of FAT filesystems is 2 seconds.
+		 */
+		private static final Duration FALLBACK_TIMESTAMP_RESOLUTION = Duration
+				.ofMillis(2000);
+
+		private static final Map<FileStore, FileStoreAttributeCache> attributeCache = new ConcurrentHashMap<>();
+
+		static Duration getFsTimestampResolution(Path file) {
+			try {
+				Path dir = Files.isDirectory(file) ? file : file.getParent();
+				if (!dir.toFile().canWrite()) {
+					// can not determine FileStore of an unborn directory or in
+					// a read-only directory
+					return FALLBACK_TIMESTAMP_RESOLUTION;
+				}
+				FileStore s = Files.getFileStore(dir);
+				FileStoreAttributeCache c = attributeCache.get(s);
+				if (c == null) {
+					c = new FileStoreAttributeCache(dir);
+					attributeCache.put(s, c);
+					if (LOG.isDebugEnabled()) {
+						LOG.debug(c.toString());
+					}
+				}
+				return c.getFsTimestampResolution();
+
+			} catch (IOException | InterruptedException e) {
+				LOG.warn(e.getMessage(), e);
+				return FALLBACK_TIMESTAMP_RESOLUTION;
+			}
+		}
+
+		private Duration fsTimestampResolution;
+
+		Duration getFsTimestampResolution() {
+			return fsTimestampResolution;
+		}
+
+		private FileStoreAttributeCache(Path dir)
+				throws IOException, InterruptedException {
+			Path probe = dir.resolve(".probe-" + UUID.randomUUID()); //$NON-NLS-1$
+			Files.createFile(probe);
+			try {
+				FileTime startTime = Files.getLastModifiedTime(probe);
+				FileTime actTime = startTime;
+				long sleepTime = 512;
+				while (actTime.compareTo(startTime) <= 0) {
+					TimeUnit.NANOSECONDS.sleep(sleepTime);
+					FileUtils.touch(probe);
+					actTime = Files.getLastModifiedTime(probe);
+					// limit sleep time to max. 100ms
+					if (sleepTime < 100_000_000L) {
+						sleepTime = sleepTime * 2;
+					}
+				}
+				fsTimestampResolution = Duration.between(startTime.toInstant(),
+						actTime.toInstant());
+			} catch (AccessDeniedException e) {
+				LOG.error(e.getLocalizedMessage(), e);
+			} finally {
+				Files.delete(probe);
+			}
+		}
+
+		@SuppressWarnings("nls")
+		@Override
+		public String toString() {
+			return "FileStoreAttributeCache[" + attributeCache.keySet()
+					.stream()
+					.map(key -> "FileStore[" + key + "]: fsTimestampResolution="
+							+ attributeCache.get(key).getFsTimestampResolution())
+					.collect(Collectors.joining(",\n")) + "]";
+		}
+	}
+
 	/** The auto-detected implementation selected for this operating system and JRE. */
 	public static final FS DETECTED = detect();
 
@@ -221,6 +306,21 @@
 		return factory.detect(cygwinUsed);
 	}
 
+	/**
+	 * Get an estimate for the filesystem timestamp resolution from a cache of
+	 * timestamp resolution per FileStore, if not yet available it is measured
+	 * for a probe file under the given directory.
+	 *
+	 * @param dir
+	 *            the directory under which the probe file will be created to
+	 *            measure the timer resolution.
+	 * @return measured filesystem timestamp resolution
+	 * @since 5.2.3
+	 */
+	public static Duration getFsTimerResolution(@NonNull Path dir) {
+		return FileStoreAttributeCache.getFsTimestampResolution(dir);
+	}
+
 	private volatile Holder<File> userHome;
 
 	private volatile Holder<File> gitSystemConfig;
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/FileUtils.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/FileUtils.java
index 97f480d..9bba6ca 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/FileUtils.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/FileUtils.java
@@ -49,6 +49,7 @@
 
 import java.io.File;
 import java.io.IOException;
+import java.io.OutputStream;
 import java.nio.file.AtomicMoveNotSupportedException;
 import java.nio.file.CopyOption;
 import java.nio.file.Files;
@@ -908,4 +909,18 @@
 		}
 		return path;
 	}
+
+	/**
+	 * Touch the given file
+	 *
+	 * @param f
+	 *            the file to touch
+	 * @throws IOException
+	 * @since 5.2.3
+	 */
+	public static void touch(Path f) throws IOException {
+		try (OutputStream fos = Files.newOutputStream(f)) {
+			// touch the file
+		}
+	}
 }