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Date: Mon, 24 Nov 2008 14:08:23 -0500 From: James Bottomley <James.Bottomley@...senPartnership.com> To: Jens Axboe <jens.axboe@...cle.com> Cc: David Woodhouse <dwmw2@...radead.org>, Matthew Wilcox <matthew@....cx>, Tejun Heo <teheo@...e.de>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Nick Piggin <npiggin@...e.de>, IDE/ATA development list <linux-ide@...r.kernel.org>, Jeff Garzik <jeff@...zik.org>, Dongjun Shin <djshin90@...il.com>, chris.mason@...cle.com Subject: Re: about TRIM/DISCARD support and barriers On Mon, 2008-11-24 at 19:57 +0100, Jens Axboe wrote: > On Mon, Nov 24 2008, David Woodhouse wrote: > > On Mon, 2008-11-24 at 13:42 -0500, James Bottomley wrote: > > > On Mon, 2008-11-24 at 09:03 +0000, David Woodhouse wrote: > > > > On Mon, 2008-11-24 at 07:52 +0900, James Bottomley wrote: > > > > > On Sun, 2008-11-23 at 13:39 +0000, David Woodhouse wrote: > > > > > > > We don't attempt to put non-contiguous ranges into a single TRIM yet. > > > > > > > > > > > > We don't even merge contiguous ranges -- I still need to fix the > > > > > > elevators to stop writes crossing writes, > > > > > > > > > > I don't think we want to do that ... it's legal if the write isn't a > > > > > barrier and it will inhibit merging. That may be just fine for a SSD, > > > > > but it's not for spinning media since they get better performance out of > > > > > merged writes. > > > > > > > > No, I just mean writes _to the same sector_. At the moment, we happily > > > > let those cross each other in the queue. > > ... > > > It's not a bug ... but changing it might be feasible ... as long as it > > > doesn't affect write performance too much (which I don't think it will), > > > since it is in the critical path. > > > > We could argue about how much sense it makes to let two writes to the > > same sector actually happen in reverse order. > > > > Especially given the fact that we actually _do_ preserve ordering in > > some cases; just not in others. (We preserve ordering only if the start > > and end of the duplicate writes are _precisely_ matching; if it's just > > overlapping (which may well happen in the presence of merges), then this > > check doesn't trigger. > > > > But that's just semantics. Yes, changing it should be feasible. I talked > > to Jens about that at the kernel summit, and we agreed that it should > > probably be done. > > The way this currently works is that we sort based on the first sector > in the request. So if you have have an overlap condition between rq1 and > rq2 and then a write gets merged into rq1, then you may have passing > writes. Linux has never guarenteed any write ordering for non-barriers, > so we've never attempted to handle it. Direct aliases (matching first > sectors) are handled as you mention, but that's more of an algorithmic > thing than by design. > > My main worry is that this will add considerable overhead to request > sorting. For the rbtree based sorting, we'd have to do a rb_next/rb_prev > to look at adjacent requests. For CFQ it's even worse, since there's no > per-queue big rbtree for sorting. Which is why I suggest special casing: Only invoke the expensive overlap checking if one of the requests is a discard. Otherwise use the standard path for writes. James -- 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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