On Mon, Aug 22, 2016 at 9:22 PM, Nick Fisk <n...@fisk.me.uk> wrote:
>> -----Original Message-----
>> From: Wido den Hollander [mailto:w...@42on.com]
>> Sent: 22 August 2016 18:22
>> To: ceph-users <ceph-users@lists.ceph.com>; n...@fisk.me.uk
>> Subject: Re: [ceph-users] udev rule to set readahead on Ceph RBD's
>>
>>
>> > Op 22 augustus 2016 om 15:17 schreef Nick Fisk <n...@fisk.me.uk>:
>> >
>> >
>> > Hope it's useful to someone
>> >
>> > https://gist.github.com/fiskn/6c135ab218d35e8b53ec0148fca47bf6
>> >
>>
>> Thanks for sharing. Might this be worth adding it to ceph-common?
>
> Maybe, Ilya kindly set the default for krbd to 4MB last year in the kernel,
> but maybe having this available would be handy if people ever want a
> different default. It could be set to 4MB as well, with a note somewhere to
> point people at its direction if they need to change it.
I remember you running tests and us talking about it, but I didn't
actually do it - the default is still a standard kernel-wide 128k.
I hesitated because it's obviously a trade off and we didn't have
a clear winner. Whatever (sensible) default we pick, users with
demanding all sequential workloads would want to crank it up anyway.
I don't have an opinion on the udev file.
>
>>
>> And is 16MB something we should want by default or does this apply to your
>> situation better?
>
> It sort of applies to me. With a 4MB readahead you will probably struggle to
> get much more than around 50-80MB/s sequential reads, as the read ahead will
> only ever hit 1 object at a time. If you want to get nearer 200MB/s then you
> need to set either 16 or 32MB readahead. I need it to stream to LTO6 tape.
> Depending on what you are doing this may or may not be required.
Thanks,
Ilya
_______________________________________________
ceph-users mailing list
ceph-users@lists.ceph.com
http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
