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From "Amitanand Aiyer (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4485) Eliminate window of missing Data
Date Thu, 20 Oct 2011 18:02:10 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4485?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13131845#comment-13131845
] 

Amitanand Aiyer commented on HBASE-4485:
----------------------------------------


Btw, Ted also has a fix to this issue where in we use a reader lock and a writer lock in Store.java,
and the writerlock is used
when we update the files. We switch back to the reader lock when we do the notifyChangedReaders.

We could definitely use that to be "safe". But, I am still trying to understand the case where
having notifyChangedReaders
outside the lock could cause problems.
                
> Eliminate window of missing Data
> --------------------------------
>
>                 Key: HBASE-4485
>                 URL: https://issues.apache.org/jira/browse/HBASE-4485
>             Project: HBase
>          Issue Type: Sub-task
>            Reporter: Amitanand Aiyer
>            Assignee: Amitanand Aiyer
>             Fix For: 0.94.0
>
>         Attachments: 4485-v1.diff, 4485-v2.diff, 4485-v3.diff, 4485-v4.diff, repro_bug-4485.diff
>
>
> After incorporating v11 of the 2856 fix, we discovered that we are still having some
ACID violations.
> This time, however, the problem is not about including "newer" updates; but, about missing
older updates
> that should be including. 
> Here is what seems to be happening.
> There is a race condition in the StoreScanner.getScanners()
>   private List<KeyValueScanner> getScanners(Scan scan,
>       final NavigableSet<byte[]> columns) throws IOException {
>     // First the store file scanners
>     List<StoreFileScanner> sfScanners = StoreFileScanner
>       .getScannersForStoreFiles(store.getStorefiles(), cacheBlocks,
>                                 isGet, false);
>     List<KeyValueScanner> scanners =
>       new ArrayList<KeyValueScanner>(sfScanners.size()+1);
>     // include only those scan files which pass all filters
>     for (StoreFileScanner sfs : sfScanners) {
>       if (sfs.shouldSeek(scan, columns)) {
>         scanners.add(sfs);
>       }
>     }
>     // Then the memstore scanners
>     if (this.store.memstore.shouldSeek(scan)) {
>       scanners.addAll(this.store.memstore.getScanners());
>     }
>     return scanners;
>   }
> If for example there is a call to Store.updateStorefiles() that happens between
> the store.getStorefiles() and this.store.memstore.getScanners(); then
> it is possible that there was a new HFile created, that is not seen by the
> StoreScanner, and the data is not present in the Memstore.snapshot either.

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