From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with ESMTP id 92F866B004F for ; Sat, 15 Aug 2009 08:59:57 -0400 (EDT) Subject: Re: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed) From: James Bottomley In-Reply-To: References: <200908122007.43522.ngupta@vflare.org> <20090813151312.GA13559@linux.intel.com> <20090813162621.GB1915@phenom2.trippelsdorf.de> <87f94c370908131115r680a7523w3cdbc78b9e82373c@mail.gmail.com> <3e8340490908131354q167840fcv124ec56c92bbb830@mail.gmail.com> <4A85E0DC.9040101@rtr.ca> <20090814234539.GE27148@parisc-linux.org> Content-Type: text/plain Date: Sat, 15 Aug 2009 07:59:36 -0500 Message-Id: <1250341176.4159.2.camel@mulgrave.site> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Chris Worley Cc: Matthew Wilcox , Mark Lord , Bryan Donlan , david@lang.hm, Greg Freemyer , Markus Trippelsdorf , Matthew Wilcox , Hugh Dickins , Nitin Gupta , Ingo Molnar , Peter Zijlstra , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-scsi@vger.kernel.org, linux-ide@vger.kernel.org, Linux RAID List-ID: On Fri, 2009-08-14 at 18:19 -0600, Chris Worley wrote: > On Fri, Aug 14, 2009 at 5:45 PM, Matthew Wilcox wrote: > > On Fri, Aug 14, 2009 at 05:21:32PM -0600, Chris Worley wrote: > >> Sooner is better than waiting to coalesce. The longer an LBA is > >> inactive, the better for any management scheme. If you wait until > >> it's reused, you might as well forgo the advantages of TRIM/UNMAP. If > >> a the controller wants to coalesce, let it coalesce. > > > > I'm sorry, you're wrong. There is a tradeoff point, and it's different > > for each drive model. Sending down a steady stream of tiny TRIMs is > > going to give terrible performance. > > Sounds like you might be using junk for a device? > > For junk, a little coalescing may be warranted... like in the I/O > schedular, but no more than 100usecs wait before posting, or then you > effect high performing devices too. Um, I think you missed the original point in all of this at the beginning of the thread: On ATA TRIM commands cannot be tagged. This means you have to drain the outstanding NCQ commands (stalling the device) before you can send a TRIM. If we do this for every discard, the performance impact will be pretty devastating, hence the need to coalesce. It's nothing really to do with device characteristics, it's an ATA protocol problem. James -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org