From: Matthew Wilcox Subject: Re: Is TRIM/DISCARD going to be a performance problem? Date: Mon, 11 May 2009 09:00:40 -0600 Message-ID: <20090511150040.GF8112@parisc-linux.org> References: <20090510165259.GA31850@logfs.org> <20090511083754.GA29082@mit.edu> <20090511100624.GB6585@logfs.org> <20090511112729.GD29082@mit.edu> <20090511120936.GB6277@mit.edu> <87f94c370905110610j2f5ea7fcua4e596b2b5e82a5f@mail.gmail.com> <20090511142740.GC6277@mit.edu> <4A08365F.5040805@redhat.com> <20090511145059.GD6277@mit.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Ric Wheeler , Greg Freemyer , J?rn Engel , Matthew Wilcox , Jens Axboe , linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, Linux RAID To: Theodore Tso Return-path: Received: from palinux.external.hp.com ([192.25.206.14]:36261 "EHLO mail.parisc-linux.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756630AbZEKPAk (ORCPT ); Mon, 11 May 2009 11:00:40 -0400 Content-Disposition: inline In-Reply-To: <20090511145059.GD6277@mit.edu> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, May 11, 2009 at 10:50:59AM -0400, Theodore Tso wrote: > On Mon, May 11, 2009 at 10:29:51AM -0400, Ric Wheeler wrote: > > The key is not at the FS layer - this is an issue for people who RAID > > these beasts together and want to actually check that the bits are what > > they should be (say doing a checksum validity check for a stripe). > > Good point, yes I can see why they need that. In that case, the > storage device can't just silently truncate a TRIM request; it would > have to expose to the OS its alignment requirements. The risk though > is that more they try push this compleixity into the OS, the higher > the risk that the OS will simply decide not to take advantage of the > functionality. Of course, there is the question why anyone would want > to build a software-raid device on top of a thin-provisioned hardware > storage unit. :-) It's not a problem for people who use Thin Provisioning, it's a problem for people who want to run RAID-5 on top of SSDs. If you have a sector whose reads are indeterminate, your parity for that stripe will always be wrong. -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step."