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 16:28:19 -0400
From:	Mark Lord <liml@....ca>
To:	Greg Freemyer <greg.freemyer@...il.com>
Cc:	Bill Davidsen <davidsen@....com>,
	James Bottomley <James.Bottomley@...e.de>,
	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)

Greg Freemyer wrote:
..
> 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.
..

Taking care of mounted RAID / LVM filesystems requires in-kernel TRIM
support, possibly exported via an ioctl().

Taking care of unmounted RAID / LVM filesystems is possible in userland,
but would also benefit from in-kernel support, where layouts are defined
and known better than in userland.

The XFS_TRIM was an idea that Cristoph floated, as a concept for examination.

I think something along those lines would be best, but perhaps with an
interface at the VFS layer.  Something that permits a userland tool
to work like this (below) might be nearly ideal:

main() {
	int fd = open(filesystem_device);
	while (1) {
		int g, ngroups = ioctl(fd, GET_NUMBER_OF_BLOCK_GROUPS);
		for (g = 0; g < ngroups; ++g) {
			ioctl(fd, TRIM_ALL_FREE_EXTENTS_OF_GROUP, g);
		}
		sleep(3600);
	}
}

Not all filesystems have a "block group", or "allocation group" structure,
but I suspect that it's an easy mapping in most cases.

With this scheme, the kernel is absolved of the need to track/coallesce
TRIM requests entirely.

Something like that, perhaps.

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