From mboxrd@z Thu Jan 1 00:00:00 1970 From: James Bottomley Subject: Re: about TRIM/DISCARD support and barriers Date: Mon, 24 Nov 2008 13:42:51 -0500 Message-ID: <1227552171.25499.30.camel@localhost.localdomain> References: <4928E010.4090801@kernel.org> <4929023C.2060302@suse.de> <20081123123514.GI5707@parisc-linux.org> <1227447584.4901.405.camel@macbook.infradead.org> <1227480776.25499.3.camel@localhost.localdomain> <1227517430.26957.20.camel@macbook.infradead.org> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: Received: from accolon.hansenpartnership.com ([76.243.235.52]:52169 "EHLO accolon.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751904AbYKXSnH (ORCPT ); Mon, 24 Nov 2008 13:43:07 -0500 In-Reply-To: <1227517430.26957.20.camel@macbook.infradead.org> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: David Woodhouse Cc: Matthew Wilcox , Tejun Heo , Linux Kernel Mailing List , Nick Piggin , Jens Axboe , IDE/ATA development list , Jeff Garzik , Dongjun Shin , chris.mason@oracle.com 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. That's legal ... if you want the ordering to matter, you either wait or insert a barrier. > We do notice this situation and preserve the ordering if the two > requests cover _precisely_ the same range, but _overlapping_ writes may > happen in any order. > > We should fix that, and it's only for _that_ purpose that I'm saying we > treat writes and discards as identical. And then we can drop the barrier > flag on discards. 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. > And _then_ we can think about special cases which let us merge > non-contiguous discards. I still think that treating discards as a special command from the outset is the better way forwards. James