[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 #4 from Manoj Pillai <mpillai at redhat.com> ---
(In reply to Manoj Pillai from comment #1)
> 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

Result with ciot=off, io-cache=off, gluster ra=on, read-ahead-page-count=10:
[these settings are based on comment #2 and comment #3]
READ: bw=975MiB/s (1023MB/s), 488MiB/s-488MiB/s (511MB/s-512MB/s), io=32.0GiB
(34.4GB)

Comparing the best results seen (1108 vs 975), the gluster ra=off case is still
a little bit better.

Result with ciot=off, io-cache=on, gluster ra=on, read-ahead-page-count=10:
READ: bw=674MiB/s (706MB/s), 337MiB/s-339MiB/s (353MB/s-355MB/s), io=32.0GiB
(34.4GB)

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