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https://issues.apache.org/jira/browse/HDFS-14941?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16967070#comment-16967070
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Chen Liang commented on HDFS-14941:
-----------------------------------
Thanks for uploading the patch [~shv]! Uploaded v02 patch.
I ran the failed tests locally, all passed except for
{{TestPipelinesFailover}}. Looks like one issue was that as part of block
recover, ANN also bumps the genstamp for the under recovery block. But since we
made the genstamp a no-op, this does not take effect on Standby. When the
Standby becomes ANN, it tries to recover the block by sending recovery command
to DN. Part of the command is the recovery ID, which is the genstamp of the new
ANN. However due to the previous no-op genstamp bump, the new ANN's genstamp is
behind what DNs expect, causing the recovery to fail.
Taking a step, in general, after the failover, ANN should catch the genstamp
created by the previous ANN. So in v02 patch, instead of having genstamp bump
as an complete no-op, its value is getting cached, but still not set to SbN
genstamp. Only when the Standby transition to ANN, it updates its genstamp to
the cached value. This way after failover, new ANN is guaranteed to see the
genstamp of old ANN, and issues new genstamp that is greater.
> Potential editlog race condition can cause corrupted file
> ---------------------------------------------------------
>
> Key: HDFS-14941
> URL: https://issues.apache.org/jira/browse/HDFS-14941
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: namenode
> Reporter: Chen Liang
> Assignee: Chen Liang
> Priority: Major
> Labels: ha
> Attachments: HDFS-14941.001.patch, HDFS-14941.002.patch
>
>
> Recently we encountered an issue that, after a failover, NameNode complains
> corrupted file/missing blocks. The blocks did recover after full block
> reports, so the blocks are not actually missing. After further investigation,
> we believe this is what happened:
> First of all, on SbN, it is possible that it receives block reports before
> corresponding edit tailing happened. In which case SbN postpones processing
> the DN block report, handled by the guarding logic below:
> {code:java}
> if (shouldPostponeBlocksFromFuture &&
> namesystem.isGenStampInFuture(iblk)) {
> queueReportedBlock(storageInfo, iblk, reportedState,
> QUEUE_REASON_FUTURE_GENSTAMP);
> continue;
> }
> {code}
> Basically if reported block has a future generation stamp, the DN report gets
> requeued.
> However, in {{FSNamesystem#storeAllocatedBlock}}, we have the following code:
> {code:java}
> // allocate new block, record block locations in INode.
> newBlock = createNewBlock();
> INodesInPath inodesInPath = INodesInPath.fromINode(pendingFile);
> saveAllocatedBlock(src, inodesInPath, newBlock, targets);
> persistNewBlock(src, pendingFile);
> offset = pendingFile.computeFileSize();
> {code}
> The line
> {{newBlock = createNewBlock();}}
> Would log an edit entry {{OP_SET_GENSTAMP_V2}} to bump generation stamp on
> Standby
> while the following line
> {{persistNewBlock(src, pendingFile);}}
> would log another edit entry {{OP_ADD_BLOCK}} to actually add the block on
> Standby.
> Then the race condition is that, imagine Standby has just processed
> {{OP_SET_GENSTAMP_V2}}, but not yet {{OP_ADD_BLOCK}} (if they just happen to
> be in different setment). Now a block report with new generation stamp comes
> in.
> Since the genstamp bump has already been processed, the reported block may
> not be considered as future block. So the guarding logic passes. But
> actually, the block hasn't been added to blockmap, because the second edit is
> yet to be tailed. So, the block then gets added to invalidate block list and
> we saw messages like:
> {code:java}
> BLOCK* addBlock: block XXX on node XXX size XXX does not belong to any file
> {code}
> Even worse, since this IBR is effectively lost, the NameNode has no
> information about this block, until the next full block report. So after a
> failover, the NN marks it as corrupt.
> This issue won't happen though, if both of the edit entries get tailed all
> together, so no IBR processing can happen in between. But in our case, we set
> edit tailing interval to super low (to allow Standby read), so when under
> high workload, there is a much much higher chance that the two entries are
> tailed separately, causing the issue.
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