[jira] [Commented] (HBASE-3845) data loss because lastSeqWritten can miss memstore edits

["ramkrishna.s.vasudevan (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-3845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13068881#comment-13068881 ] 

ramkrishna.s.vasudevan commented on HBASE-3845:
-----------------------------------------------

I have submitted the patch for 0.90.4 version.
I ran all the testcases.  It was ok.
But automating this particular scenario through test case was not feasible.
Please review and provide your comments. 
I can submit for trunk also. 

> 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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