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https://issues.apache.org/jira/browse/HBASE-11099?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13985237#comment-13985237
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stack commented on HBASE-11099:
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[~jeffreyz] On the first, what do you see as the hole? There are no writes
coming in when I take the snapshot but there may be edits queued in the ring
buffer that have yet to take a seqid? These edits may be in the snapshot so
could be included in the hfile but because we took the flushSeqId before they
finished, these 'extra' edits could be overwritten on HRegion open? Hard to
know when my regions' edits have all cleared the ring buffer. I could add in a
sync and wait till it completes but that could take a while if the ring buffer
filled by other regions writing. Let me see....
> Two situations where we could open a region with smaller sequence number
> ------------------------------------------------------------------------
>
> Key: HBASE-11099
> URL: https://issues.apache.org/jira/browse/HBASE-11099
> Project: HBase
> Issue Type: Bug
> Components: regionserver
> Affects Versions: 0.99.0
> Reporter: Jeffrey Zhong
> Fix For: 0.99.0
>
>
> Recently I happened to run into code where we potentially could open region
> with smaller sequence number:
> 1) Inside function: HRegion#internalFlushcache. This is due to we change the
> way WAL Sync where we use late binding(assign sequence number right before
> wal sync).
> The flushSeqId may less than the change sequence number included in the flush
> which may cause later region opening code to use a smaller than expected
> sequence number when we reopen the region.
> {code}
> flushSeqId = this.sequenceId.incrementAndGet();
> ...
> mvcc.waitForRead(w);
> {code}
> 2) HRegion#replayRecoveredEdits where we have following code:
> {code}
> ...
> if (coprocessorHost != null) {
> status.setStatus("Running pre-WAL-restore hook in coprocessors");
> if (coprocessorHost.preWALRestore(this.getRegionInfo(), key,
> val)) {
> // if bypass this log entry, ignore it ...
> continue;
> }
> }
> ...
> currentEditSeqId = key.getLogSeqNum();
> {code}
> If coprocessor skip some tail WALEdits, then the function will return smaller
> currentEditSeqId. In the end, a region may also open with a smaller sequence
> number. This may cause data loss because Master may record a larger flushed
> sequence Id and some WALEdits maybe skipped during recovery if the region
> fail again.
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