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 08:56:12 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	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 (was Re: [PATCH] swap: send callback when swap
 slot is freed)

On Sun, 2009-08-16 at 12:59 -0400, Christoph Hellwig wrote:
> On Sun, Aug 16, 2009 at 10:52:07AM -0500, James Bottomley wrote:
> > However, the enterprise has been doing UNMAP for a while, so we can draw
> > inferences from them since the SSD FTL will operate similarly.  For
> > them, UNMAP is the same cost in terms of time regardless of the number
> > of extents.  The reason is that it's moving the blocks from the global
> > in use list to the global free list.  Part of the problem is that this
> > involves locking and quiescing, so UNMAP ends up being quite expensive
> > to the array but constant in terms of cost (hence they want as few
> > unmaps for as many sectors as possible).
> 
> How are they doing the unmaps?  Using something similar to Mark's wiper
> script and using SG_IO?  Because right now we do not actually implement
> UNMAP support in the kernel.  I'd really love to test the XFS batched
> discard support with a real UNMAP implementation.

You mean how is the array vendor testing their implementation?  Using
SG_IO ... without any filesystem, I believe.

The testing was initially done to see if the initial maximal discard
proposal from LSF09 was a viable approach (which it wasn't given the
time taken to UNMAP).

James


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