[Bugs] [Bug 1665029] read-ahead and io-cache degrading performance on sequential read

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1665029



--- Comment #2 from Raghavendra G <rgowdapp at redhat.com> ---
Some observations while debugging the performance degradation with gluster
read-ahead:

* Kernel too does read-ahead and it sends parallel read-requests as part of
this.
* client-io-threads is on in this configuration.

The above two points mean parallel requests sent by kernel can reach read-ahead
out of order. This means read-ahead no longer sees read requests at sequential
contiguous offsets and hence it things reads are random. For random reads, it
resets the read sequence. But when requests reach read-ahead in order,
read-ahead is turned on again. Due to this intermittent toggling, much of
read-ahead data is wasted regressing the performance. With client-io-threads
off and I can no longer see the regression for the test case given. If I run
the test with single fio job (--numjobs=1), gluster read-ahead on outperforms
gluster read-ahead off on my setup.

[1] https://review.gluster.org/#/c/glusterfs/+/20981/

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