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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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