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https://issues.apache.org/jira/browse/HBASE-15871?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15502358#comment-15502358
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Anoop Sam John commented on HBASE-15871:
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WHich previous? Checking the readpnt and avoid call to Memstore scanner create? 
I would say that should be avoided..  Create it and close the scanner if we are 
sure no cells are going to come from it..  The master patch is having many null 
check on peeked cells..  But u found an issue with close state.. Ya once that 
is fixed we can see how/whether we can address that..  So branch-1 also similar 
fix can be done no?

> Memstore flush doesn't finish because of backwardseek() in memstore scanner.
> ----------------------------------------------------------------------------
>
>                 Key: HBASE-15871
>                 URL: https://issues.apache.org/jira/browse/HBASE-15871
>             Project: HBase
>          Issue Type: Bug
>          Components: Scanners
>    Affects Versions: 1.1.2
>            Reporter: Jeongdae Kim
>            Assignee: ramkrishna.s.vasudevan
>             Fix For: 2.0.0, 1.4.0
>
>         Attachments: HBASE-15871-branch-1.patch, 
> HBASE-15871.branch-1.1.001.patch, HBASE-15871.branch-1.1.002.patch, 
> HBASE-15871.branch-1.1.003.patch, HBASE-15871.patch, HBASE-15871_1.patch, 
> HBASE-15871_1.patch, HBASE-15871_2.patch, HBASE-15871_3.patch, 
> HBASE-15871_4.patch, memstore_backwardSeek().PNG
>
>
> Sometimes in our production hbase cluster, it takes a long time to finish 
> memstore flush.( for about more than 30 minutes)
> the reason is that a memstore flusher thread calls 
> StoreScanner.updateReaders(), waits for acquiring a lock that store scanner 
> holds in StoreScanner.next() and backwardseek() in memstore scanner runs for 
> a long time.
> I think that this condition could occur in reverse scan by the following 
> process.
> 1) create a reversed store scanner by requesting a reverse scan.
> 2) flush a memstore in the same HStore.
> 3) puts a lot of cells in memstore and memstore is almost full.
> 4) call the reverse scanner.next() and re-create all scanners in this store 
> because all scanners was already closed by 2)'s flush() and backwardseek() 
> with store's lastTop for all new scanners.
> 5) in this status, memstore is almost full by 2) and all cells in memstore 
> have sequenceID greater than this scanner's readPoint because of 2)'s 
> flush(). this condition causes searching all cells in memstore, and 
> seekToPreviousRow() repeatly seach cells that are already searched if a row 
> has one column. (described this in more detail in a attached file.)
> 6) flush a memstore again in the same HStore, and wait until 4-5) process 
> finished, to update store files in the same HStore after flusing.
> I searched HBase jira. and found a similar issue. (HBASE-14497) but, 
> HBASE-14497's fix can't solve this issue because that fix just changed 
> recursive call to loop.(and already applied to our HBase version)



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