linux-kernel - Re: libata: set queue SSD flag for SSD devices

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Message-ID: <20081011140600.GR19428@kernel.dk>
Date:	Sat, 11 Oct 2008 16:06:01 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-ide@...r.kernel.org, torvalds@...l.org
Subject: Re: libata: set queue SSD flag for SSD devices

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@...cle.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

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