[Devel] Re: IO scheduler based IO controller V10

[Corrado Zoccolo]

On Fri, Oct 2, 2009 at 9:58 PM, Vivek Goyal <vgoyal at redhat.com> wrote:
> On Fri, Oct 02, 2009 at 12:50:17PM -0400, Valdis.Kletnieks at vt.edu wrote:
>> On Fri, 02 Oct 2009 11:40:20 EDT, Vivek Goyal said:
>>
>> Umm... I got petabytes of hardware RAID across the hall that very definitely
>> *is* rotating.  Did you mean "SSD and disk systems with big honking caches
>> that cover up the rotation"?  Because "RAID" and "big honking caches" are
>> not *quite* the same thing, and I can just see that corner case coming out
>> to bite somebody on the ass...
>>
>
> I guess both. The systems which have big caches and cover up for rotation,
> we probably need not idle for seeky process. An in case of big hardware
> RAID, having multiple rotating disks, instead of idling and keeping rest
> of the disks free, we probably are better off dispatching requests from
> next queue (hoping it is going to a different disk altogether).

In fact I think that the 'rotating' flag name is misleading.
All the checks we are doing are actually checking if the device truly
supports multiple parallel operations, and this feature is shared by
hardware raids and NCQ enabled SSDs, but not by cheap SSDs or single
NCQ-enabled SATA disk.

If we really wanted a "seek is cheap" flag, we could measure seek time
in the io-scheduler itself, but in the current code base we don't have
it used in this meaning anywhere.

Thanks,
Corrado

>
> Thanks
> Vivek
>



-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo at gmail.com
PhD - Department of Computer Science - University of Pisa, Italy
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