Re: [DO NOT APPLY] sd take advantage of rotation speed

[Andrew Patterson <andrew.patterson@hp.com>]

On Thu, 2008-07-31 at 14:00 -0700, Grant Grundler wrote:
> On Mon, Jul 28, 2008 at 7:31 AM, Martin K. Petersen
> <martin.petersen@oracle.com> wrote:
> >>>>>> "Ric" == Ric Wheeler <rwheeler@redhat.com> writes:
> >
> > Ric> One other thought - is there a way to give non-rotational devices
> > Ric> also some indication of latency? (FLASH is slower than enterprise
> > Ric> SSD is slower than DRAM ramdisk for example)?
> >
> > The current SBC draft only distinguishes between rotating media
> > speeds.  There is only one classification for non-rotating media in
> > the block device characteristics VPD.
> >
> > For a mechanical disk drive the rpm isn't a terrible gauge for
> > performance.  But for a solid state device I think it will be hard to
> > define a similar universal metric.
> 
> rpm isn't a great gauge of performance either since the perf is a
> function of rpm * bit density.
> 
> > Ignoring SLC vs. MLC for a moment I think it's also safe to predict
> > that the enterprise drive of today will be the consumer drive of
> > tomorrow.
> >
> > Maybe the ssd device could export the anticipated command response
> > time for a request that matches the Optimal Transfer Length field in
> > the block limits VPD?
> 
> erase and/or write times could be exported as well somehow for SSDs
> if the FS (or other higher layer that wants to know) can't avoid
> garbage collection and erase cycles. I was just told today that flash devices
> have 10x higher write time than read time. erase is another order of
> magnitude higher. This doesn't include any garbage collection overhead.
> 
> I think new file systems should be tuned to work with SSDs before we
> worry so much about the differences between SSDs/flash technologies
> and vendors. And then prescribe a different FS for different
> storage technologies. This avoids the "layering violations" discussion
> and helps keep the FSs (testing and developement) substantially simpler.
> 

Could performance patterns be encoded in some sort of per-product data
structure ala the scsi_static_device_list?

Andrwe