[Bugs] [Bug 1601166] performance.read-ahead causes huge increase in unnecessary network traffic

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1601166

Worker Ant <bugzilla-bot at gluster.org> changed:

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             Status|POST                        |MODIFIED



--- Comment #4 from Worker Ant <bugzilla-bot at gluster.org> ---
COMMIT: https://review.gluster.org/20510 committed in master by "Raghavendra G"
<rgowdapp at redhat.com> with a commit message- performance/read-ahead: stricter
adherence to force-atime-update

Throwaway read-ahead cache in fstat only if force-atime-update is
set. Note that fstat flushes read-ahead cache only for atime
consistency. However if atime consistency is needed user is required
to set force-atime-update which updates atime on backend fs even
though application reads are served from read-ahead cache. So, if user
has not set force-atime-update, atime won't be accurate and there is
no point in flushing read-ahead cache in fstats. mounts
requiring atime consistency have to mandatorily set
force-atime-update.

Also note that normally kernel interspers reads with fstat. So,
read-ahead is not effective as fstats flush read-ahead-cache. Instead
it regresses performance due to wasted network reads. It is
recommended to turn off read-ahead if applications require atime
consistency.

This patch is aimed at applications which don't require atime
consistency. Without atime consistency required, read-ahead cache is
effective and increases performance of sequential reads.

Change-Id: I122bbc410cee96661823f9c4b934383495c18446
Signed-off-by: Raghavendra G <rgowdapp at redhat.com>
Fixes: bz#1601166

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