[Gluster-devel] On making performance.parallel-readdir as a default option

[Raghavendra Gowdappa]

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:
* 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
<https://bugzilla.redhat.com/show_bug.cgi?id=1628807#c34>
[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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