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Posted to issues@hbase.apache.org by "ramkrishna.s.vasudevan (JIRA)" <ji...@apache.org> on 2011/07/21 11:40:58 UTC
[jira] [Updated] (HBASE-3845) data loss because lastSeqWritten can
miss memstore edits
[ https://issues.apache.org/jira/browse/HBASE-3845?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
ramkrishna.s.vasudevan updated HBASE-3845:
------------------------------------------
Attachment: HBASE-3845_1.patch
> data loss because lastSeqWritten can miss memstore edits
> --------------------------------------------------------
>
> Key: HBASE-3845
> URL: https://issues.apache.org/jira/browse/HBASE-3845
> Project: HBase
> Issue Type: Bug
> Affects Versions: 0.90.3
> Reporter: Prakash Khemani
> Assignee: ramkrishna.s.vasudevan
> Priority: Critical
> Fix For: 0.90.4
>
> Attachments: HBASE-3845_1.patch
>
>
> (I don't have a test case to prove this yet but I have run it by Dhruba and Kannan internally and wanted to put this up for some feedback.)
> In this discussion let us assume that the region has only one column family. That way I can use region/memstore interchangeably.
> After a memstore flush it is possible for lastSeqWritten to have a log-sequence-id for a region that is not the earliest log-sequence-id for that region's memstore.
> HLog.append() does a putIfAbsent into lastSequenceWritten. This is to ensure that we only keep track of the earliest log-sequence-number that is present in the memstore.
> Every time the memstore is flushed we remove the region's entry in lastSequenceWritten and wait for the next append to populate this entry again. This is where the problem happens.
> step 1:
> flusher.prepare() snapshots the memstore under HRegion.updatesLock.writeLock().
> step 2 :
> as soon as the updatesLock.writeLock() is released new entries will be added into the memstore.
> step 3 :
> wal.completeCacheFlush() is called. This method removes the region's entry from lastSeqWritten.
> step 4:
> the next append will create a new entry for the region in lastSeqWritten(). But this will be the log seq id of the current append. All the edits that were added in step 2 are missing.
> ==
> as a temporary measure, instead of removing the region's entry in step 3 I will replace it with the log-seq-id of the region-flush-event.
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