-Re: probable there is a bug in HLog Implementation
Anty 2011-05-20, 13:22
Can you take some time to take a look at it.
On Fri, May 20, 2011 at 1:04 PM, Anty <[EMAIL PROTECTED]> wrote:
> 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
> 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