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 13:08:23 -0400
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Bill Davidsen <davidsen@....com>
Cc:	James Bottomley <James.Bottomley@...e.de>, 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)

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.

For the other known cases:

SSDs that meet the ata-8 spec, but don't exceed it
Enterprise SCSI
mdraid with SSD storage used to build raid5 / raid6 arrays

Non-coalescing is believed detrimental, 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.

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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