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https://issues.apache.org/jira/browse/AMQ-4987?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13879805#comment-13879805
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anselme dewavrin commented on AMQ-4987:
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Dear All,
The iowait and huge disk activity is due to frequent fsyncs on the slaves. I
demonstrated this by preloading a library that disables fsyncs (with a
LD_PRELOAD=libeatmydata.so). With this trick the iowait disappear.
I think the leveldb replicated stores is in itself secure enough without
fsync-ing each update, because of the synchronous replication (sync=quorum_mem)
on different machines that should fail within the same seconds. This is why the
many fsyncs on the slaves are not useful, in my opinion.
For my purpose I will live with the LD_PRELOAD, but it could be profitable for
the community to make the levelDB replicated store evolve.
Anselme
> io wait on replicated levelDB slaves
> ------------------------------------
>
> Key: AMQ-4987
> URL: https://issues.apache.org/jira/browse/AMQ-4987
> Project: ActiveMQ
> Issue Type: Test
> Components: activemq-leveldb-store
> Affects Versions: 5.9.0
> Environment: debian VM 2.6.32-5-amd64, jdk7
> Reporter: anselme dewavrin
> Priority: Minor
>
> Dear all,
> I set up a 3-nodes replicatedLevelDB activeMQ cluster as explained on the
> activemq site.
> I made a message injector using the php stomp client described here :
> http://stomp.fusesource.org/documentation/php/book.html
> Then I injected persistent messages as fast as possible (giving about 600
> messages/s).
> Everything works fine, then I measured the servers' activity with "vmstat 1".
> I saw no iowait on the master node, but 20% on both slaves. This would
> impeach scalabitity I suppose. And the iowait is justified by 3000 bo/s
> (blocks out) in the vmstat report.
> The machines are not swapping (paging).
> Here is what I tried, without success :
> -specify explicitly sync="quorum_mem"
> -JNI implementation of the leveldb store (and verified it is used)
> -setting flushDelay to 2000
> Does anyone have an idea that I could try ? Why is the leveldb slaves writing
> so much to disk ?
> Many thanks in advance
> Yours,
> Anselme
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