Hi:All
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.
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Best Regards
Anty Rao
