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From "Prakash Khemani (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-3845) data loss because lastSeqWritten can miss memstore edits
Date Mon, 02 May 2011 20:01:04 GMT
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
            Reporter: Prakash Khemani


(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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