diff --git a/src/main/java/com/googlesource/gerrit/plugins/findowners/Action.java b/src/main/java/com/googlesource/gerrit/plugins/findowners/Action.java
index d0c642f..493ae7e 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/findowners/Action.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/findowners/Action.java
@@ -67,6 +67,7 @@
 
   static class Parameters {
     Boolean debug; // REST API "debug" parameter, or null
+    Boolean nocache; // REST API "nocache" parameter, or null
     Integer patchset; // REST API "patchset" parameter, or null
   }
 
@@ -104,7 +105,7 @@
 
   private List<OwnerInfo> getOwners(OwnersDb db, Collection<String> files) {
     Map<String, OwnerWeights> weights = new HashMap<>();
-    db.findOwners(files, weights);
+    db.findOwners(files, weights, new ArrayList<>());
     List<OwnerInfo> result = new ArrayList<>();
     Set<String> emails = new HashSet<>();
     for (String key : OwnerWeights.sortKeys(weights)) {
@@ -182,9 +183,9 @@
       throws OrmException, BadRequestException, IOException {
     int patchset = getValidPatchsetNum(changeData, params.patchset);
     ProjectState projectState = projectCache.get(changeData.project());
-    OwnersDb db =
-        Cache.getInstance()
-            .get(projectState, accountCache, emails, repository, changeData, patchset);
+    Boolean useCache = params.nocache == null || !params.nocache;
+    OwnersDb db = Cache.getInstance().get(
+        useCache, projectState, accountCache, emails, repository, changeData, patchset);
     Collection<String> changedFiles = changeData.currentFilePaths();
     Map<String, Set<String>> file2Owners = db.findOwners(changedFiles);
 
@@ -201,6 +202,7 @@
       obj.dbgmsgs.errors = db.errors;
       obj.dbgmsgs.path2owners = Util.makeSortedMap(db.path2Owners);
       obj.dbgmsgs.owner2paths = Util.makeSortedMap(db.owner2Paths);
+      obj.dbgmsgs.logs = db.logs;
     }
 
     obj.file2owners = Util.makeSortedMap(file2Owners);
@@ -236,6 +238,7 @@
           OwnersDb db =
               Cache.getInstance()
                   .get(
+                      true, // use cached OwnersDb
                       projectCache.get(resource.getProject()),
                       accountCache,
                       emails,
diff --git a/src/main/java/com/googlesource/gerrit/plugins/findowners/Cache.java b/src/main/java/com/googlesource/gerrit/plugins/findowners/Cache.java
index f1a2bee..bf01666 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/findowners/Cache.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/findowners/Cache.java
@@ -88,6 +88,7 @@
 
   /** Returns a cached or new OwnersDb, for the current patchset. */
   OwnersDb get(
+      Boolean useCache,
       ProjectState projectState,
       AccountCache accountCache,
       Emails emails,
@@ -95,6 +96,7 @@
       ChangeData changeData)
       throws OrmException, IOException {
     return get(
+        useCache,
         projectState,
         accountCache,
         emails,
@@ -105,6 +107,7 @@
 
   /** Returns a cached or new OwnersDb, for the specified patchset. */
   OwnersDb get(
+      Boolean useCache,
       ProjectState projectState,
       AccountCache accountCache,
       Emails emails,
@@ -116,6 +119,7 @@
     String dbKey = Cache.makeKey(changeData.getId().get(), patchset, branch);
     // TODO: get changed files of the given patchset?
     return get(
+        useCache,
         projectState,
         accountCache,
         emails,
@@ -128,6 +132,7 @@
 
   /** Returns a cached or new OwnersDb, for the specified branch and changed files. */
   OwnersDb get(
+      Boolean useCache,
       ProjectState projectState,
       AccountCache accountCache,
       Emails emails,
@@ -136,7 +141,7 @@
       ChangeData changeData,
       String branch,
       Collection<String> files) {
-    if (dbCache == null) { // Do not cache OwnersDb
+    if (dbCache == null || !useCache) { // Do not cache OwnersDb
       logger.atFiner().log("Create new OwnersDb, key=%s", key);
       return new OwnersDb(
           projectState, accountCache, emails, key, repository, changeData, branch, files);
diff --git a/src/main/java/com/googlesource/gerrit/plugins/findowners/Checker.java b/src/main/java/com/googlesource/gerrit/plugins/findowners/Checker.java
index 2d71283..e361f82 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/findowners/Checker.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/findowners/Checker.java
@@ -151,7 +151,7 @@
     // One update to a Gerrit change can call submit_rule or submit_filter
     // many times. So this function should use cached values.
     OwnersDb db =
-        Cache.getInstance().get(projectState, accountCache, emails, repository, changeData);
+        Cache.getInstance().get(true, projectState, accountCache, emails, repository, changeData);
     if (db.getNumOwners() <= 0) {
       return 0;
     }
diff --git a/src/main/java/com/googlesource/gerrit/plugins/findowners/GetOwners.java b/src/main/java/com/googlesource/gerrit/plugins/findowners/GetOwners.java
index c62a909..5d19399 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/findowners/GetOwners.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/findowners/GetOwners.java
@@ -46,6 +46,10 @@
   @Option(name = "--debug", usage = "get extra debug info")
   private String debug;
 
+  // "nocache" could be true/yes/1 or false/no/0, default is false.
+  @Option(name = "--nocache", usage = "do not use cached owners info")
+  private String nocache;
+
   @Option(name = "--patchset", usage = "select change patchset number")
   private Integer patchset;
 
@@ -78,6 +82,7 @@
     Action.Parameters params = new Action.Parameters();
     params.patchset = patchset;
     params.debug = (debug != null) ? Util.parseBoolean(debug) : null;
+    params.nocache = (nocache != null) ? Util.parseBoolean(nocache) : null;
     try {
       return this.action.apply(rsrc, params);
     } catch (BadRequestException e) {
diff --git a/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersDb.java b/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersDb.java
index 227369a..edd1a0a 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersDb.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersDb.java
@@ -23,11 +23,14 @@
 import com.google.gerrit.server.account.Emails;
 import com.google.gerrit.server.project.ProjectState;
 import com.google.gerrit.server.query.change.ChangeData;
+import java.net.InetAddress;
+import java.net.UnknownHostException;
 import java.nio.file.FileSystem;
 import java.nio.file.FileSystems;
 import java.nio.file.PathMatcher;
 import java.nio.file.Paths;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -58,6 +61,7 @@
   Set<String> stopLooking = new HashSet<>(); // directories where OWNERS has "set noparent"
   Map<String, String> preferredEmails = new HashMap<>(); // owner email to preferred email
   List<String> errors = new ArrayList<>(); // error messages
+  List<String> logs = new ArrayList<>(); // trace/debug messages
 
   OwnersDb() {}
 
@@ -73,10 +77,18 @@
     this.accountCache = accountCache;
     this.emails = emails;
     this.key = key;
+    try {
+      InetAddress inetAddress = InetAddress.getLocalHost();
+      logs.add("HostName:" + inetAddress.getHostName());
+    } catch (UnknownHostException e) {
+      logException(logs, "HostName:", e);
+    }
+    logs.add("key:" + key);
     preferredEmails.put("*", "*");
     String ownersFileName = Config.getOwnersFileName(projectState, changeData);
+    logs.add("ownersFileName:" + ownersFileName);
     // Some hacked CL could have a target branch that is not created yet.
-    ObjectId id = getBranchId(repository, branch, changeData);
+    ObjectId id = getBranchId(repository, branch, changeData, logs);
     revision = "";
     if (id != null) {
       for (String fileName : files) {
@@ -84,10 +96,12 @@
         // Stop looking for a parent directory if OWNERS has "set noparent".
         fileName = Util.normalizedFilePath(fileName);
         String dir = Util.normalizedDirPath(fileName); // e.g. dir = ./d1/d2
+        logs.add("findOwnersFileFor:" + fileName);
         while (!readDirs.contains(dir)) {
           readDirs.add(dir);
+          logs.add("findOwnersFileIn:" + dir);
           String filePath = (dir + "/" + ownersFileName).substring(2); // remove "./"
-          String content = getRepositoryFile(repository, id, filePath);
+          String content = getRepositoryFile(repository, id, filePath, logs);
           if (content != null && !content.equals("")) {
             addFile(dir + "/", dir + "/" + ownersFileName, content.split("\\R+"));
           }
@@ -102,6 +116,7 @@
       } catch (Exception e) {
         logger.atSevere().withCause(e).log(
             "Fail to get branch revision for %s", Config.getChangeId(changeData));
+        logException(logs, "OwnersDb get revision", e);
       }
     }
     countNumOwners(files);
@@ -112,7 +127,8 @@
   }
 
   private void countNumOwners(Collection<String> files) {
-    Map<String, Set<String>> file2Owners = findOwners(files, null);
+    logs.add("countNumOwners");
+    Map<String, Set<String>> file2Owners = findOwners(files, null, logs);
     if (file2Owners != null) {
       Set<String> emails = new HashSet<>();
       file2Owners.values().forEach(emails::addAll);
@@ -141,6 +157,7 @@
         email2ids = emails.getAccountsFor(ownerEmailsAsArray);
       } catch (Exception e) {
         logger.atSevere().withCause(e).log("accounts.byEmails failed");
+        logException(logs, "getAccountsFor:" + ownerEmailsAsArray[0], e);
       }
       for (String owner : ownerEmailsAsArray) {
         String email = owner;
@@ -192,8 +209,10 @@
       ArrayList<Integer> distances,
       String file,
       Map<String, Set<String>> file2Owners,
-      Map<String, OwnerWeights> map) {
+      Map<String, OwnerWeights> map,
+      List<String> logs) {
     for (int i = 0; i < paths.size(); i++) {
+      logs.add("addOwnerWeightsIn:" + paths.get(i));
       Set<String> owners = path2Owners.get(paths.get(i));
       if (owners == null) {
         continue;
@@ -214,13 +233,13 @@
 
   /** Quick method to find owner emails of every file. */
   Map<String, Set<String>> findOwners(Collection<String> files) {
-    return findOwners(files, null);
+    return findOwners(files, null, new ArrayList<>());
   }
 
   /** Returns owner emails of every file and set up ownerWeights. */
   Map<String, Set<String>> findOwners(
-      Collection<String> files, Map<String, OwnerWeights> ownerWeights) {
-    return findOwners(files.toArray(new String[0]), ownerWeights);
+      Collection<String> files, Map<String, OwnerWeights> ownerWeights, List<String> logs) {
+    return findOwners(files.toArray(new String[0]), ownerWeights, logs);
   }
 
   /** Returns true if path has '*' owner. */
@@ -238,14 +257,18 @@
   }
 
   /** Returns owner emails of every file and set up ownerWeights. */
-  Map<String, Set<String>> findOwners(String[] files, Map<String, OwnerWeights> ownerWeights) {
+  Map<String, Set<String>> findOwners(
+      String[] files, Map<String, OwnerWeights> ownerWeights, List<String> logs) {
     // Returns a map of file to set of owner emails.
     // If ownerWeights is not null, add to it owner to distance-from-dir;
     // a distance of 1 is the lowest/closest possible distance
     // (which makes the subsequent math easier).
+    logs.add("findOwners");
+    Arrays.sort(files); // Force an ordered search sequence.
     Map<String, Set<String>> file2Owners = new HashMap<>();
     for (String fileName : files) {
       fileName = Util.normalizedFilePath(fileName);
+      logs.add("checkFile:" + fileName);
       String dirPath = Util.normalizedDirPath(fileName);
       String baseName = fileName.substring(dirPath.length() + 1);
       int distance = 1;
@@ -257,6 +280,7 @@
       boolean foundStar = false;
       while (true) {
         int savedSizeOfPaths = paths.size();
+        logs.add("checkDir:" + dirPath);
         if (dir2Globs.containsKey(dirPath + "/")) {
           Set<String> patterns = dir2Globs.get(dirPath + "/");
           for (String pat : patterns) {
@@ -287,40 +311,59 @@
         dirPath = Util.getDirName(dirPath); // go up one level
       }
       if (!foundStar) {
-        addOwnerWeights(paths, distances, fileName, file2Owners, ownerWeights);
+        addOwnerWeights(paths, distances, fileName, file2Owners, ownerWeights, logs);
+      } else {
+        logs.add("found * in:" + fileName);
       }
     }
     return file2Owners;
   }
 
   /** Returns ObjectId of the given branch, or null. */
-  private static ObjectId getBranchId(Repository repo, String branch, ChangeData changeData) {
+  private static ObjectId getBranchId(
+      Repository repo, String branch, ChangeData changeData, List<String> logs) {
+    String header = "getBranchId:" + branch;
     try {
       ObjectId id = repo.resolve(branch);
       if (id == null && changeData != null && !Checker.isExemptFromOwnerApproval(changeData)) {
         logger.atSevere().log(
             "cannot find branch %s for %s", branch, Config.getChangeId(changeData));
+        logs.add(header + " (NOT FOUND)");
+      } else {
+        logs.add(header + " (FOUND)");
       }
       return id;
     } catch (Exception e) {
       logger.atSevere().withCause(e).log(
           "cannot find branch %s for %s", branch, Config.getChangeId(changeData));
+      logException(logs, header, e);
     }
     return null;
   }
 
   /** Returns file content or empty string; uses Repository. */
-  private static String getRepositoryFile(Repository repo, ObjectId id, String file) {
+  private static String getRepositoryFile(
+      Repository repo, ObjectId id, String file, List<String> logs) {
+    String header = "getRepositoryFile:" + file;
     try (RevWalk revWalk = new RevWalk(repo)) {
       RevTree tree = revWalk.parseCommit(id).getTree();
       ObjectReader reader = revWalk.getObjectReader();
       TreeWalk treeWalk = TreeWalk.forPath(reader, file, tree);
       if (treeWalk != null) {
-        return new String(reader.open(treeWalk.getObjectId(0)).getBytes(), UTF_8);
+        String content = new String(reader.open(treeWalk.getObjectId(0)).getBytes(), UTF_8);
+        logs.add(header + ":" + content);
+        return content;
       }
+      logs.add(header + " (NOT FOUND)");
     } catch (Exception e) {
       logger.atSevere().withCause(e).log("get file %s", file);
+      logException(logs, header, e);
     }
     return "";
   }
+
+  /** Adds a header + exception message to the logs. */
+  private static void logException(List<String> logs, String header, Exception e) {
+    logs.add(header + " Exception:" + e.getMessage());
+  }
 }
diff --git a/src/main/java/com/googlesource/gerrit/plugins/findowners/RestResult.java b/src/main/java/com/googlesource/gerrit/plugins/findowners/RestResult.java
index 15f01de..378f2de 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/findowners/RestResult.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/findowners/RestResult.java
@@ -57,5 +57,6 @@
     List<String> errors;
     SortedMap<String, List<String>> path2owners;
     SortedMap<String, List<String>> owner2paths;
+    List<String> logs;
   }
 }
diff --git a/src/main/resources/Documentation/rest-api.md b/src/main/resources/Documentation/rest-api.md
index 10b1565..d287480 100644
--- a/src/main/resources/Documentation/rest-api.md
+++ b/src/main/resources/Documentation/rest-api.md
@@ -19,6 +19,8 @@
 * **debug**: can be set to true or false to override the configuration variable
   **addDebugMsg**.
 
+* **nocache**: can be set to true to collect owerns info without using the cached OwnersDb.
+
 For example,
 
 ```bash
@@ -66,6 +68,9 @@
       a map from owner email to an array of directory path or file glob.
       This is opposite to the path2owners map.
 
+    * **logs**:
+      trace messages during the search of OWNERS files.
+
 * **file2owners**: a map from each file path in the change patchset to
    an array of the file's owner emails.
 
diff --git a/src/test/java/com/googlesource/gerrit/plugins/findowners/FindOwnersIT.java b/src/test/java/com/googlesource/gerrit/plugins/findowners/FindOwnersIT.java
index 323a861..6198139 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/findowners/FindOwnersIT.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/findowners/FindOwnersIT.java
@@ -96,8 +96,11 @@
     assertThat(getOwnersResponse(c1)).contains(ownersAX + ", files:[ OWNERS ]");
     // Check all fields in response.
     String expectedTail =
-        "path2owners:{ ./:[ a@a, x@x ] }, owner2paths:{ a@a:[ ./ ], x@x:[ ./ ] } }"
-            + ", file2owners:{ ./t.c:[ a@a, x@x ] }, reviewers:[], "
+        "path2owners:{ ./:[ a@a, x@x ] }, owner2paths:{ a@a:[ ./ ], x@x:[ ./ ] }, logs:[ "
+            + "key:12:1:master, ownersFileName:OWNERS, getBranchId:refs/heads/master(FOUND), "
+            + "findOwnersFileFor:./t.c, findOwnersFileIn:., getRepositoryFile:OWNERS:x@x\\na@a\\n, "
+            + "countNumOwners, findOwners, checkFile:./t.c, checkDir:., addOwnerWeightsIn:./ ] "
+            + "}, file2owners:{ ./t.c:[ a@a, x@x ] }, reviewers:[], "
             + ownersAX
             + ", files:[ t.c ] }";
     assertThat(getOwnersDebugResponse(c2)).contains(expectedTail);
@@ -454,14 +457,15 @@
     Repository repo = repoManager.openRepository(project);
     Cache cache = Cache.getInstance().init(0, 0);
     OwnersDb db =
-        cache.get(projectCache.get(project), accountCache, emails, repo, r.getChange(), 1);
+        cache.get(true, projectCache.get(project), accountCache, emails, repo, r.getChange(), 1);
     Checker c = new Checker(repo, r.getChange(), 1);
     return c.findApproval(accountCache, db);
   }
 
-  // Remove '"' and space; replace '\n' with ' '; ignore unpredictable "owner_revision".
+  // Remove '"' and space; replace '\n' with ' '; ignore "owner_revision" and "HostName:*".
   private static String filteredJson(String json) {
-    return json.replaceAll("[\" ]*", "").replace('\n', ' ').replaceAll("owner_revision:[^ ]* ", "");
+    return json.replaceAll("[\" ]*", "").replace('\n', ' ').replaceAll("owner_revision:[^ ]* ", "")
+        .replaceAll("HostName:[^ ]*, ", "");
   }
 
   private static String filteredJson(RestResponse response) throws Exception {