linux-kernel - Re: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed)

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Date:	Mon, 17 Aug 2009 17:19:35 +0000
From:	James Bottomley <James.Bottomley@...e.de>
To:	Greg Freemyer <greg.freemyer@...il.com>
Cc:	Bill Davidsen <davidsen@....com>, Mark Lord <liml@....ca>,
	Arjan van de Ven <arjan@...radead.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Chris Worley <worleys@...il.com>,
	Matthew Wilcox <matthew@....cx>,
	Bryan Donlan <bdonlan@...il.com>, david@...g.hm,
	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 (was Re: [PATCH] swap: send callback when swap
  slot is freed)

On Mon, 2009-08-17 at 13:08 -0400, Greg Freemyer wrote:
> All,
> 
> Seems like the high-level wrap-up of all this is:
> 
> There are hopes that highly efficient SSDs will appear on the market
> that can leverage a passthru non-coalescing discard feature.  And that
> a whitelist should be created to allow those SSDs to see discards
> intermixed with the rest of the data i/o.

That's not my conclusion.  Mine was the NCQ drain would still be
detremental to interleaved trim even if the drive could do it for zero
cost.

> For the other known cases:
> 
> SSDs that meet the ata-8 spec, but don't exceed it
> Enterprise SCSI

No, SCSI will do WRITE_SAME/UNMAP as currently drafted in SBC3

> mdraid with SSD storage used to build raid5 / raid6 arrays
> 
> Non-coalescing is believed detrimental,

It is?  Why?

>  but a regular flushing of the
> unused blocks/sectors via a tool like Mark Lord has written should be
> acceptable.
> 
> Mark, I don't believe your tool really addresses the mdraid situation,
> do you agree.  ie. Since your bypassing most of the block stack,
> mdraid has no way of snooping on / adjusting the discards you are
> sending out.
> 
> Thus the 2 solutions that have been worked on already seem to address
> the needs of everything but mdraid.

I count three:  Mark Lord script via SG_IO.  hch enhanced script via
XFS_TRIM and willy current discard inline which he's considering
coalescing for.

James

> Also, there has been no discussion of dm based volumes.  (ie LVM2 based volumes)
> 
> For mdraid or dm it seems we need to enhance Mark's script to pass the
> trim commands through the full block stack.  Mark, please cmiiw
> 
> Greg


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