ramkrishna.s.vasudevan commented on HBASE-15871:

I can change it. Thanks Ted for the review.
Tried adding a 'closed' flag in SegmentScanner and to see how things work and 
found an interesting bug. So if we need to go with 'closed' flag in 
SegmentScanner and check for the flag on every operation then we need to solve 
the new bug. It is quite critical too. 

> 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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