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 14:16:19 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	James Bottomley <James.Bottomley@...e.de>
CC:	Greg Freemyer <greg.freemyer@...il.com>,
	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)

Chiming in here a bit late, but coalescing requests is also a good way 
to prevent read-modify-write cycles.

Specifically, if I remember the concern correctly, for the WRITE_SAME 
with unmap bit set, when the IO is not evenly aligned on the "erase 
chunk" (whatever they call it) boundary the device can be forced to do a 
read-modify-write (of zeroes) to the end or beginning of that region.

For a disk array, the WRITE_SAME with unmap bit when done cleanly on an 
aligned boundary can be done entirely in the array's cache. The 
read-modify-write can generate several reads to the back end disks which 
are significantly slower....

ric

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