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From "Feng Honghua (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HBASE-10556) Possible data loss due to non-handled DroppedSnapshotException for user-triggered flush from client/shell
Date Mon, 17 Feb 2014 08:27:19 GMT

     [ https://issues.apache.org/jira/browse/HBASE-10556?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Feng Honghua updated HBASE-10556:
---------------------------------

    Status: Patch Available  (was: Open)

> Possible data loss due to non-handled DroppedSnapshotException for user-triggered flush
from client/shell
> ---------------------------------------------------------------------------------------------------------
>
>                 Key: HBASE-10556
>                 URL: https://issues.apache.org/jira/browse/HBASE-10556
>             Project: HBase
>          Issue Type: Bug
>          Components: regionserver
>            Reporter: Feng Honghua
>            Assignee: Feng Honghua
>            Priority: Critical
>         Attachments: HBASE-10556-trunk_v1.patch
>
>
> During the code review when investigating HBASE-10499, a possibility of data loss due
to non-handled DroppedSnapshotException for user-triggered flush is exposed.
> Data loss can happen as below:
> # A flush for some region is triggered via HBaseAdmin or shell
> # The request reaches regionserver and eventually HRegion.internalFlushcache is called,
then fails at persisting memstore's snapshot to hfile, DroppedSnapshotException is thrown
and the snapshot is left not cleared.
> # DroppedSnapshotException is not handled in HRegion, and is just encapsulated as a ServiceException
before returning to client
> # After a while, some new writes are handled and put in the current memstore, then a
new flush is triggered for the region due to memstoreSize exceeds flush threshold
> # This second(new) flush succeeds, for the HStore which failed in the previous user-triggered
flush, the remained non-empty snapshot is used rather than a new snapshot made from the current
memstore, but HLog's latest sequenceId is used for the resultant hfiles --- the sequenceId
attached within the hfiles says all edits with sequenceId <= it have all been persisted,
but actually it's not the truth for the edits still in the existing memstore
> # Now the regionserver hosting this region dies
> # During the replay phase of failover, the edits corresponding to the ones while are
in memstore and not actually persisted in hfiles when the previous regionserver dies will
be ignored, since they are deemed as persisted by compared to the hfiles' latest consequenceID
--- These edits are lost...
> For the second flush, we also can't discard the remained snapshot and make a new one
using current memstore, that way the data in the remained snapshot is lost. We should abort
the regionserver immediately and rely on the failover to replay the log for data safety.
> DroppedSnapshotException is correctly handled in MemStoreFlusher for internally triggered
flush (which are generated by flush-size / rollWriter / periodicFlusher). But user-triggered
flush is processed directly by HRegionServer->HRegion without putting a flush entry to
flushQueue, hence not handled by MemStoreFlusher



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