[Bugs] [Bug 1094328] poor fio rand read performance

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1094328

Raghavendra G <rgowdapp at redhat.com> changed:

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--- Comment #13 from Raghavendra G <rgowdapp at redhat.com> ---
I had a discussion with Manoj on the numbers got by Mohit. Things puzzling to
me and Manoj are:

* fio is run in O_DIRECT mode, which means read-ahead is turned off
*  we've a bug where read-ahead won't work when open-behind is turned on [2].
Since open-behind is on by default, read-ahead should not be functioning at
all.

Since we've two reasons to believe read-ahead is switched off, how are we
seeing improvement in benchmarks?

Another point to note is that the jobfile for fio Mohit used doesn't exactly
generate strided reads. Instead it just generates random reads. Manoj suggested
following to generate strided reads with fio

<mpillai>       raghug, there seems to a straighforward way of generating
strided pattern with fio. you'd use sequential io instead of rand, but give a
stride. e.g. the option rw=read means seq read. rw=read:128k means read with a
stride of 128k. that's what a quick read of the howto is suggesting.

Also, note that patch [1] doesn't exactly implement an optimization for strided
read pattern (If it was, it had to be comparing delta's of previous reads with
current read). It just keeps the cache if current read-region is already cached
by read-ahead. So, we need to decide on whether to have more stringent strided
read-ahead implementation or its beneficial to just keep the implementation of
[1].

[1] https://review.gluster.org/#/c/7676
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1084508

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