Re: libata: set queue SSD flag for SSD devices

Arjan van de Ven
2008-10-11T16:49:50Z

On Sat, 11 Oct 2008 18:38:44 +0200
James Bottomley <James.Bottomley@HansenPartnership.com> wrote:

> On Sat, 2008-10-11 at 09:04 -0700, Arjan van de Ven wrote:
> > On Sat, 11 Oct 2008 17:44:13 +0200
> > James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> >
> > >
> > > > So we need something a bit more involved, but not too complex. A
> > > > fine line...
> > >
> > > It's a policy ... just let userspace do it so the user can tune
> > > it. That's what EMC does now (except I think they key of inquiry
> > > strings rather than cache size).
> >
> >
> > while the chosen elevator obviously is policy, the kernel really
> > should pick a sensible default based on what it knows.
> > Lets put it this way: if userland needs to do a tuning to the kernel
> > based on data only provided by the kernel, and will always do it the
> > same way, we should have made that choice the default policy in the
> > kernel in the first place.
>
> Well, this is a bit of a nasty layering problem. We certainly don't
> want the Block layer to know how to poke at SATA, SCSI and other
> esoteric media to see what elevator should be the default, so we'd
> have to craft a new block API that the lower subsystems would
> implement for this. I'm really not sure it's worth the trouble when
> the boot system can do it simply from userspace, but I'll defer to
> Jens.
>

these devices already give the elevator layer information about the
device, like optimal/max io size etc.
having the elevator take a "don't bother optimizing for seeks" flag
is very much along the same lines, it's a device property that the
elevator needs to learn about in order to send the right kinds of IO
down.

--
Arjan van de Ven Intel Open Source Technology Centre
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