Alan,
I've tried 'echo "1"> /sys/block/<dsf>/queue/nomerges' but fio/sequential-reads merges still occur and apparently they are only the simple ones that can't be disabled.
1) What can I change in the kernel to disable ALL the merges (including the simple ones)?
(I tried setting iosched_cfq.ops.elevator_merge_fn, elevator_merged_fn, elevator_merge_req_fn, elevator_allow_merge_fn = 0 but still fio reported merges)
2) One other question I have is why one driver benefits of the io scheduler merges and the other doesn't?
(Note that HDD's don't have any other io activity - aside from fio - and both are doing the same 512B sequential reads)
3) The third question is why the number of fio reported "merge"(s) is greater than the number of the "ios"?
thanks,
John
--- On Mon, 1/4/10, Alan D. Brunelle <Alan.Brunelle@xxxxxx> wrote:
> From: Alan D. Brunelle <Alan.Brunelle@xxxxxx>
> Subject: Re: io scheduler merges control
> To: "john smith" <whalajam@xxxxxxxxx>
> Cc: fio@xxxxxxxxxxxxxxx
> Date: Monday, January 4, 2010, 6:34 AM
> On Thu, 2009-12-24 at 11:12 -0800,
> john smith wrote:
> > IOPS-reported numbers for a test vary (more than 30%
> in my case) from one run to another depending on kernel io
> scheduler merges (reported in "Disk
> stats/merges=reads/writes") making it hard to
> measure/compare performance of block drivers, for instance.
> >
> > I don't see any way to turn off kernel io scheduler
> merges (cfq or others)
> > and I'd ask if you would consider adding support for
> it, disabling the merges per individual device, using sys-fs
> maybe?
> >
> > If you do, adding an option to fio for controlling the
> merges would be useful too,
> >
> > thanks,
> > John
>
> Hi John -
>
> FYI: there is /sys/block/*/queue/nomerges - when set to one
> ('echo 1
> > /sys/block/<dsf>/queue/nomerges) - a lot of the
> merging attempts are
> not performed. [Note, _some_ simple merge attempts _are_
> still attempted
> regardless of its setting...]
>
> Regards,
> Alan
>
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