From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with SMTP id 44B9F6B004F for ; Fri, 14 Aug 2009 20:30:44 -0400 (EDT) Received: by qyk36 with SMTP id 36so1562271qyk.12 for ; Fri, 14 Aug 2009 17:30:42 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: References: <200908122007.43522.ngupta@vflare.org> <20090813162621.GB1915@phenom2.trippelsdorf.de> <87f94c370908131115r680a7523w3cdbc78b9e82373c@mail.gmail.com> <3e8340490908131354q167840fcv124ec56c92bbb830@mail.gmail.com> <4A85E0DC.9040101@rtr.ca> <20090814234539.GE27148@parisc-linux.org> Date: Fri, 14 Aug 2009 20:30:42 -0400 Message-ID: <87f94c370908141730y3ddcb7bbj65d24b612fc0e96d@mail.gmail.com> Subject: Re: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed) From: Greg Freemyer Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org To: Chris Worley Cc: Matthew Wilcox , Mark Lord , Bryan Donlan , david@lang.hm, 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, Aug 14, 2009 at 8:19 PM, 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. =A0The longer an LBA is >>> inactive, the better for any management scheme. =A0If you wait until >>> it's reused, you might as well forgo the advantages of TRIM/UNMAP. =A0I= f >>> a the controller wants to coalesce, let it coalesce. >> >> I'm sorry, you're wrong. =A0There is a tradeoff point, and it's differen= t >> for each drive model. =A0Sending 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. > > Chris Why? AIUI, on every write a high performing device allocates a new erase block from its free lists, writes to it, and puts the now unused erase block on the free list. That erase block becomes available for reuse some milliseconds later. As long as the SSD has enough free erase blocks to work with I see no disadvantage in delaying a discard by minutes, hours or days in most cases. The exception is when the filesystem is almost full and the SSD is short of erase blocks to work with. In that case it will want to get as many free erase blocks as it can as fast as it can get them. Greg -- 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