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From Anty <anty....@gmail.com>
Subject probable there is a bug in HLog Implementation
Date Fri, 20 May 2011 05:04:25 GMT
     after reading source code of HLog, i'm wandering wheather it's a bug.
     for example, only one region is active. max log size is a fraction of
region size.
     flush begins, region A acquire a sequecne number,say, N.
     insert operation can continue while we flush the cache.
     flush opeartion complete, delete region A's entry in lastSeqWritten(Map
of regions to most recet sequence/edit id in their memstore)
     when flush compelte, current sequence number maybe N+5, five log
messages added to the log for region A during the flush operation .
     region A going on to accept update, insert a new entry into
lastSeqWritten for region A, but in current HLog implementation the value
is  N+6 .
     But i tink the value corresponding to Region A in lastSeqWritten should
be N,not N+6.
     N+6 means all edits whose sequence number smaller than N+6  in Region A
is already persisent on disk, but it's not the fact.
     edits N+1,N+2,N+3,N+4,N+5, the new five edit are maybe in memstore of
Region A.
     So, the value should be N, the sequence number when flush begins and
flush completes.
     the above procedure leave a change of data loss.
     though in current implementation the chance of data loss is rare.
     So,i think it's a bug.
     the fix is easy, when flush complete, just set the value for Region A
in lastSeqWritten to N instead of removing the entry .
     if you want a data loss scenario, i can you give you one.

     if i miss something , Pls let me known.

Best Regards
Anty Rao

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