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From Dhruba Borthakur <dhr...@gmail.com>
Subject Re: commit semantics
Date Tue, 12 Jan 2010 06:25:38 GMT
any IO to a HDFS-file (appends, writes, etc) ae actually blocked on a
pending sync. "sync" in HDFS is a pretty heavyweight operation as it stands.

if we want the best of both worlds.. latency as well as data integrity, how
about inserting the same record into two completely separate HBase tables in
parallel... the operation can complete as soon as the record is inserted
into the first HBase table (thus giving low latencies) but data integrity
will not be compromised because it is unlikely that two region servers will
fail exactly at the same time (assuming that there is a way to ensure that
these two tables are not handled by the same region server).

thanks,
dhruba


On Mon, Jan 11, 2010 at 8:12 PM, Joydeep Sarma <jssarma@apache.org> wrote:

> ok - hadn't thought about it that way - but yeah with a default of 1 -
> the semantics seem correct.
>
> under high load - some batching would automatically happen at this
> setting (or so one would think - not sure if hdfs appends are blocked
> on pending syncs (in which case the batching wouldn't quite happen i
> think) - cc'ing Dhruba).
>
> if the performance with setting of 1 doesn't work out - we may need an
> option to delay acks until actual syncs .. (most likely we would be
> able to compromise on latency to get higher throughput - but wouldn't
> be willing to compromise on data integrity)
>
> > Hey Joydeep,
> >
> > This is actually intended this way but the name of the variable is
> > misleading. The sync is done only if forceSync or we have enough
> > entries to sync (default is 1). If someone wants to sync only 100
> > entries for example, they would play with that configuration.
> >
> > Hope that helps,
> >
> > J-D
> >
> >
> > On Mon, Jan 11, 2010 at 3:46 PM, Joydeep Sarma <jssarma@apache.org>
> wrote:
> >>
> >> Hey HBase-devs,
> >>
> >> we have been going through hbase code to come up to speed.
> >>
> >> One of the questions was regarding the commit semantics. Thumbing
> through the RegionServer code that's appending to the wal:
> >>
> >> syncWal -> HLog.sync -> addToSyncQueue ->syncDone.await()
> >>
> >> and the log writer thread calls:
> >>
> >> hflush(), syncDone.signalAll()
> >>
> >> however hflush doesn't necessarily call a sync on the underlying log
> file:
> >>
> >>       if (this.forceSync ||
> >>           this.unflushedEntries.get() >= this.flushlogentries) { ...
> sync() ... }
> >>
> >> so it seems that if forceSync is not true, the syncWal can unblock
> before a sync is called (and forcesync seems to be only true for
> metaregion()).
> >>
> >> are we missing something - or is there a bug here (the signalAll should
> be conditional on hflush having actually flushed something).
> >>
> >> thanks,
> >>
> >> Joydeep
> >
>



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