Re: libata: set queue SSD flag for SSD devices

[Jens Axboe <jens.axboe@oracle.com>]

On Sat, Oct 11 2008, James Bottomley wrote:
> On Fri, 2008-10-10 at 17:55 -0700, Arjan van de Ven wrote:
> > On Fri, 10 Oct 2008 22:05:28 +0200
> > Jens Axboe <jens.axboe@oracle.com> wrote:
> > > 
> > > In this specific patch, it'll do no harm at least since I very much
> > > doubt we'll see a false positive. And even if, the consequences wont
> > > be dire. But it does want the version check, of course.
> > > 
> > 
> > also, is "is an ssd" the right question or is "doesn't have seek
> > latency" the right one?
> > (difference is.. well EMC boxes with lots of ram etc)
> 
> For libata, the is ssd is the right test.
> 
> For things like the symmetrix we probably need to look at the caching
> mode page and treat it like ssd if the cache is say > 1GB or whatever
> heuristic is needed.

Still wont quite work, you're still going to have occasional seek
latencies no matter how large the cache is (unless it's == device size
:-)

So we need something a bit more involved, but not too complex. A fine
line...

-- 
Jens Axboe