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https://issues.apache.org/jira/browse/COUCHDB-271?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Jan Lehnardt updated COUCHDB-271:
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Fix Version/s: (was: 1.4)
> preventing compaction from ruining the OS block cache
> -----------------------------------------------------
>
> Key: COUCHDB-271
> URL: https://issues.apache.org/jira/browse/COUCHDB-271
> Project: CouchDB
> Issue Type: Improvement
> Components: Database Core
> Affects Versions: 0.8.1, 0.9
> Reporter: Jan Lehnardt
>
> Adam Kocolosk:
> Hi, I've noticed that compacting large DBs pretty much kills any filesystem
> caching benefits for CouchDB. I believe the problem is that the OS (Linux
> 2.6.21 kernel in my case) is caching blocks from the .compact file, even
> though those blocks won't be read again until compaction has finished. In
> the meantime, the portion of the cache dedicated to the old DB file shrinks
> and performance really suffers.
> I think a better mode of operation would be to advise/instruct the OS not to
> cache any portion of the .compact file until we're ready to replace the main
> DB. On Linux, specifying the POSIX_FADV_DONTNEED option to posix_fadvise()
> seems like the way to go:
> http://linux.die.net/man/2/posix_fadvise
> This link has a little more detail and a usage example:
> http://insights.oetiker.ch/linux/fadvise.html
> Of course, POSIX_FADV_DONTNEED isn't really available from inside the Erlang
> VM. Perhaps the simplest approach would be to have a helper process that we
> can spawn which calls that function (or its equivalent on a non-Linux OS)
> periodically during compaction? I'm not really sure, but I wanted to get
> this out on the list for discussion. Best,
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