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From "Jan Lehnardt (JIRA)" <>
Subject [jira] Created: (COUCHDB-271) preventing compaction from ruining the OS block cache
Date Sat, 28 Feb 2009 09:04:12 GMT
preventing compaction from ruining the OS block cache

                 Key: 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:

This link has a little more detail and a usage example:

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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