From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mark Lord <liml@rtr.ca>
Subject: Re: Discard support (was Re: [PATCH] swap: send callback when swap
 	slot is freed)
Date: Mon, 17 Aug 2009 16:28:19 -0400
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To: Greg Freemyer <greg.freemyer@gmail.com>
Cc: Bill Davidsen <davidsen@tmr.com>, James Bottomley <James.Bottomley@suse.de>, Arjan van de Ven <arjan@infradead.org>, Alan Cox <alan@lxorguk.ukuu.org.uk>, Chris Worley <worleys@gmail.com>, Matthew Wilcox <matthew@wil.cx>, Bryan Donlan <bdonlan@gmail.com>, david@lang.hm, Markus Trippelsdorf <markus@trippelsdorf.de>, Matthew Wilcox <willy@linux.intel.com>, Hugh Dickins <hugh.dickins@tiscali.co.uk>, Nitin Gupta <ngupta@vflare.org>, Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-scsi@vger.kernel.org, linux-ide@vger.kernel.org, Linux RAID <linux-raid@vger.kernel.org>
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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.