[GitHub] [ozone] hemantk-12 commented on a diff in pull request #4909: HDDS-8832. [Snapshot] Synchronized add snapshotInfo to snapshot chain and snapshot cache

["hemantk-12 (via GitHub)" <gi...@apache.org>]

hemantk-12 commented on code in PR #4909:
URL: https://github.com/apache/ozone/pull/4909#discussion_r1231401641


##########
hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/SnapshotChainManager.java:
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@@ -48,6 +47,10 @@
 public class SnapshotChainManager {
   private static final Logger LOG =
       LoggerFactory.getLogger(SnapshotChainManager.class);
+  // Dummy lock to synchronize addSnapshot and deleteSnapshot.
+  // This lock is also used by OMSnapshotCreateRequest to make sure that
+  // snapshot gets added to chain only if addition to cache is successfully.
+  public static final Object LOCK = new Object();

Review Comment:
   This lock is also used by `OMSnapshotCreateRequest`.
   I tried to explain it https://github.com/apache/ozone/pull/4909/files#diff-36b9bd9ebbbb4522cfe1ed312e68ea2ac91e9934a92d146ebb52a96644978e49R229-R231 and https://github.com/apache/ozone/pull/4909/files#diff-36b9bd9ebbbb4522cfe1ed312e68ea2ac91e9934a92d146ebb52a96644978e49R205-R220.
   
   I couldn't think of better idea. Even if I synchronize snapshot creation separately in `OMSnapshotCreateRequest`, it won't be sufficient. Because [deleteSnapshot](https://github.com/apache/ozone/blob/master/hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/SnapshotChainManager.java#L272) of  `SnapshotChainManager` can be called by `OMSnapshotPurgeResponse`. Which will leave us in similar situation like https://github.com/apache/ozone/pull/4909/files#diff-36b9bd9ebbbb4522cfe1ed312e68ea2ac91e9934a92d146ebb52a96644978e49R210-R220 if snapshot gets purge at the same time new gets created.



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