On Mon, Mar 13, 2017 at 11:00 AM, Dan van der Ster <dan@xxxxxxxxxxxxxx> wrote:
>> I'm sorry, I may have worded that in a manner that's easy to
>> misunderstand. I generally *never* suggest that people use CFQ on
>> reasonably decent I/O hardware, and thus have never come across any
>> need to set this specific ceph.conf parameter.
>
> OTOH, cfq *does* help our hammer clusters. deadline's default
> behaviour is to delay writes up to 5 seconds if the disk is busy
> reading -- which it is, of couse, while deep scrubbing. And deadline
> does not offer any sort of fairness between processes accessing the
> same disk (which is admittedly less of an issue in jewel). But back in
> hammer days it was nice to be able to make the disk threads only read
> while the disk was otherwise idle.
Thanks for pointing out the default 5000-ms write deadline. We
frequently tune that down to 1500ms. Disabling front merges also
sometimes seems to help.
For the archives: those settings are in
/sys/block/*/queue/iosched/{write_expire,front_merges} and can be
persisted on Debian/Ubuntu with sysfsutils.
Cheers,
Florian
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