linux-kernel - Re: swap storage alignment and stride size

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Message-ID: <alpine.LSU.2.00.1012142038430.13004@tigran.mtv.corp.google.com>
Date:	Tue, 14 Dec 2010 20:57:28 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
cc:	Ric Wheeler <ricwheeler@...il.com>,
	Christian Brandt <brandtc@...5.com>,
	linux-kernel@...r.kernel.org, Mike Snitzer <snitzer@...hat.com>
Subject: Re: swap storage alignment and stride size

On Tue, 14 Dec 2010, Martin K. Petersen wrote:
> >>>>> "Ric" == Ric Wheeler <ricwheeler@...il.com> writes:
> 
> Ric> There has been a lot of work on alignment, Martin Petersen lead
> Ric> most of that and is probably the best one to ping.
> 
> With modern tooling we should align the partition or DM device correctly
> so the swap starts on a properly aligned boundary. But I don't think
> anybody has looked into hooking the swap stuff up with the I/O
> topology. I'm also not sure the swap code is flexible enough to deal
> with units that are bigger than page size.

You and Christian are right, mm/swapfile.c is very much oriented to
the small mm page size, 4kB on x86.

Yes, when it's running nicely, the elevator can make a big difference
by merging adjacent writes to swap; but swapping is often by nature
not so nice.

I think it would be a big mistake to try to build the idea of bigger
blocks into mm/swapfile.c: it is so orientated towards the mm concerns
that we'd end up with a mess that way.

Much better to add a dm layer below it, to buffer such alignment and
stride concerns.  Perhaps someone has already done that?

(scan_swap_map does try to allocate in 1MB clusters, but they're not
written out that way, and there's no attempt to align: if it worked
out better for the lower level to require that these 1MB clusters
are aligned, we could probably go for that - though the swap header
page might then be a nuisance.)

Hugh
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