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https://issues.apache.org/jira/browse/COUCHDB-271?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12743898#action_12743898
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Robert Newson commented on COUCHDB-271:
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Another way to approach this is to eliminate wholesale database writing to
achieve compaction.
Specifically, instead of a single file for a couchdb database it would be an
ordered sequence of files. It's still append-only, so earlier files will
contain data thats been superceded by updates, etc, just as they do today. Each
file is eligible to be compacted separately by reading all the extant records
from it and writing them to the end of the current file, the old file is then
deleted. With this approach (c.f. Berkeley JE), compaction could be an ongoing
background task, would not require 100% as much disk space as the database
itself, and the current inability to swap to the .compact file in the presence
of constant writes would also be addressed.
> 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
> Fix For: 0.10
>
>
> 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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