Hi all,
We've
a feature performance.parallel-readdir [1] that is known to improve
performance of readdir operations [2][3][4]. The option is especially
useful when distribute scale is relatively large (>10) and is known
to improve performance of readdir operations even on smaller scale of
distribute count 1 [4].However, this option is not enabled by default. I am here proposing to make this as a default feature.
But,
there are some important things to be addressed in readdir-ahead (which
is core part of parallel-readdir), before we can do so:
To summarize issues with readdir-ahead:
To summarize issues with readdir-ahead:
* There
seems to be one prominent problem of missing dentries with
parallel-readdir. There was one problem discussed on tech-list just
yesterday. I've heard about this recurrently earlier too. Not sure
whether this is the problem of missing unlink/rmdir/create etc fops (see
below) in readdir-ahead. ATM, no RCA.
* fixes to maintain stat-consistency in dentries pre-fetched have not made into downstream yet (though merged upstream [5]).
*
readdir-ahead doesn't implement directory modification fops like
rmdir/create/symlink/link/unlink/rename. This means cache won't be
updated wiith newer content, even on single mount till its
consumed by application or purged.
* dht linkto-files should
store relative positions of subvolumes instead of absolute subvolume
name, so that changes to immediate child won't render them stale.*
Features parallel-readdir depends on to be working should be enabled
automatically even though they were off earlier when parallel-readdir is
enabled [6].
I've listed important known issues above. But we can discuss which are the blockers for making this feature as a default.
Thoughts?
[1] http://review.gluster.org/#/c/16090/
[2] https://events.static.linuxfound.org/sites/events/files/slides/Gluster_DirPerf_Vault2017_0.pdf (sections on small directory)
[3] https://bugzilla.redhat.com/show_bug.cgi?id=1628807#c35
[4] https://www.spinics.net/lists/gluster-users/msg34956.html
[5] http://review.gluster.org/#/c/glusterfs/+/20639/
[6] https://bugzilla.redhat.com/show_bug.cgi?id=1631406
regards,
Raghavendra
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