[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 #1 from Manoj Pillai <mpillai at redhat.com> ---

Data showing that both read-ahead and io-cache cause performance degradation.
I'm modifying the test in comment #0 here, substituting numjobs=2, instead of
4.

Test sequence:
fio --name=initialwrite --ioengine=sync --rw=write --direct=0
--create_on_open=1 --end_fsync=1 --bs=128k --directory=/mnt/glustervol/
--filename_format=f.\$jobnum.\$filenum --filesize=16g --size=16g --numjobs=2
[unmount and mount volume]
fio --name=readtest --ioengine=sync --rw=read --direct=0 --bs=128k
--directory=/mnt/glustervol/ --filename_format=f.\$jobnum.\$filenum
--filesize=16g --size=16g --numjobs=2

Result with default settings:
READ: bw=485MiB/s (509MB/s), 243MiB/s-243MiB/s (254MB/s-255MB/s), io=32.0GiB
(34.4GB), run=67504-67522msec

Result with read-ahead turned off:
READ: bw=776MiB/s (813MB/s), 388MiB/s-388MiB/s (407MB/s-407MB/s), io=32.0GiB
(34.4GB), run=42220-42237msec

Result with read-ahead and io-cache turned off:
READ: bw=1108MiB/s (1162MB/s), 554MiB/s-554MiB/s (581MB/s-581MB/s), io=32.0GiB
(34.4GB), run=29565-29573msec

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