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

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Message-ID: <4A88BAF5.2000009@rtr.ca>
Date:	Sun, 16 Aug 2009 22:05:41 -0400
From:	Mark Lord <liml@....ca>
To:	John Robinson <john.robinson@...nymous.org.uk>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	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)

John Robinson wrote:
..
> I'm slightly out of my depth here, but: if a single TRIM is issued, 
> which apparently returns quickly, can one then revert to issuing 
> ordinary commands like reads and writes and have them complete as 
> quickly as they normally do, or does any following command have to wait 
> until the trim completes? This could be useful if it turned out we won't 
> stall these devices as long as we don't issue more than one TRIM every 
> few seconds; we could keep a TRIM coalesce queue down to being (say) 5 
> seconds long (or at least, a configurable small number of seconds).
..

I have not attempted to instrument that, but I suspect that any
command after the TRIM has to wait.  Don't know for sure until
somebody measures it though.

One thing I do know, is that Matthew's first cut of TRIM support
means it takes half an hour to do "rm -r" on a kernel source tree.

-ml
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