linux-kernel - Re: Discard support

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Message-ID: <4A888D6B.8020103@rtr.ca>
Date:	Sun, 16 Aug 2009 18:51:23 -0400
From:	Mark Lord <liml@....ca>
To:	Theodore Tso <tytso@....edu>, Roland Dreier <rdreier@...co.com>,
	James Bottomley <James.Bottomley@...e.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, Mark Lord <liml@....ca>,
	Chris Worley <worleys@...il.com>,
	Matthew Wilcox <matthew@....cx>,
	Bryan Donlan <bdonlan@...il.com>, david@...g.hm,
	Greg Freemyer <greg.freemyer@...il.com>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Nitin Gupta <ngupta@...are.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	Linux RAID <linux-raid@...r.kernel.org>
Subject: Re: Discard support

Theodore Tso wrote:
> On Sun, Aug 16, 2009 at 02:50:40PM -0700, Roland Dreier wrote:
>>  > Well, yes and no ... a lot of SSDs don't actually implement NCQ, so the
>>  > impact to them will be less ... although I think enterprise class SSDs
>>  > do implement NCQ.
>>
>> Really?  Which SSDs don't implement NCQ?
> 
> The Intel X25-M was the first SSD to implement NCQ.  The OCZ Core V2
> advertised NCQ with a queue depth of 1, but even that was buggy, so
> Linux has a black list for the that SSD:
> 
>       http://markmail.org/message/jvjpmcdqjwrmyl4w
> 
> As far as I know, all of the SSD's using the crappy JMicron JMF602
> controllers don't support NCQ in any real way, which would includes
> most of the reasonably priced SSD's up until first half of this year.
> (The OCZ Summit SSD, which uses the Indilnx controller is an exception
> to this statement, but it's more expensive that the JMF602 based
> SSD's, although less expensive than the Intel SSD.)
> 
> JMicron is trying to seek redemption with their JMF612 controllers,
> which were announced at the end of May of this year, and those
> controllers do support NCQ, and are claimed to not to have the
> disatrous small write latency problem of their '602 bretheren.  I'm
> not aware of any products using the JMF612 yet, though.  (According to
> reports the '612 controllers weren't going to hit mass production
> until July, so hopefully later this fall we'll start seeing products
> using the new JMicron controller.)
..

Great summary, Ted.

To add:  SSDs based upon the Indilinx controller don't appear to scale
beyond an NCQ depth of about 4 or so.  Whereas Intel SSDs continue to
improve with increased queue depth up to 31 on Linux.

Or at least that's what I recall from reading various SSD benchmarks
a few weeks ago here. 

Cheers
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