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

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Message-ID: <4A8814A0.4040708@rtr.ca>
Date:	Sun, 16 Aug 2009 10:16:00 -0400
From:	Mark Lord <liml@....ca>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	James Bottomley <James.Bottomley@...e.de>,
	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)

Alan Cox wrote:
>
> Flush barriers are nightmare for more than enterprise. You drive
> basically goes for a hike for a bit which trashes interactivity as well.
> If the device can't do trim and the like without a drain I don't see much
> point doing it at all, except maybe to wait for idle devices and run a
> filesystem managed background 'strimmer' thread to just weed out now idle
> blocks that have stayed idle - eg by adding an inode of all the deleted
> untrimmed blocks and giving it an irregular empty ?
..

Agreed.  And I believe Matthew also said something similar already.
TRIM for the current (only!) SSDs needs to be a "batched, once in a while"
operation, rather than something done continuously.

And ideally, "once in a while" might be once a day, or once we have more
than a significant percentage of the drive capacity ready for a TRIM.

It needs to batch a lot of stuff into a single TRIM,
and not do it very often at all.

Cheers
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