From: Ric Wheeler Subject: Re: Is TRIM/DISCARD going to be a performance problem? Date: Mon, 11 May 2009 10:58:38 -0400 Message-ID: <4A083D1E.2040807@redhat.com> 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=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Greg Freemyer , =?ISO-8859-1?Q?J=F6rn_Enge?= =?ISO-8859-1?Q?l?= , Matthew Wilcox , Jens Axboe , linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, Linux RAID To: Theodore Tso Return-path: In-Reply-To: <20090511145059.GD6277@mit.edu> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 05/11/2009 10:50 AM, 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. :-) > > - Ted Probably not as uncommon as you would think, but not as you suggest to raid thin provisioned luns (those are done usually as RAID devices inside an array). Think more of the array providing a thinly provisioned LUN made up out of T13 TRIM enabled SSD's devices internally. RAID makes sense here (data protection is still needed to avoid a single point of failure) and the relative expense of the SSD's devices makes "thin provisioning" really attractive to external users :-) ric