From: Edward Shishkin Subject: Re: [PATCH 2/2] Add batched discard support for ext4 - using rbtree Date: Tue, 27 Apr 2010 17:29:06 +0200 Message-ID: <4BD702C2.7000209@redhat.com> References: <1271674527-2977-2-git-send-email-lczerner@redhat.com> <1271674527-2977-3-git-send-email-lczerner@redhat.com> <4BCE6243.5010209@teksavvy.com> <4BCE66C5.3060906@redhat.com> <4BCF4C53.3010608@redhat.com> <20100426165527.GB21179@atrey.karlin.mff.cuni.cz> <4BD5D2EA.1070008@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Jeff Moyer , Ric Wheeler , Jan Kara , Greg Freemyer , Eric Sandeen , Mark Lord , linux-ext4@vger.kernel.org, Edward Shishkin , Eric Sandeen , Christoph Hellwig To: Lukas Czerner Return-path: Received: from mx1.redhat.com ([209.132.183.28]:1025 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752703Ab0D0P3R (ORCPT ); Tue, 27 Apr 2010 11:29:17 -0400 In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: Lukas Czerner wrote: > On Mon, 26 Apr 2010, Lukas Czerner wrote: > > >> Create an ioctl which walks through all the free extents in each >> allocating group and discard those extents. As an addition to improve >> its performance one can specify minimum free extent length, so ioctl >> will not bother with shorter extents. >> >> This of course means, that with each invocation the ioctl must walk >> through whole file system, checking and discarding free extents, which >> is not very efficient. The best way to avoid this is to keep track of >> deleted (freed) blocks. Then the ioctl have to trim just those free >> extents which were recently freed. >> >> In order to implement this I have created new structure >> ext4_deleted_data which represents deleted extent in per-group rbtree. >> When blocks are freed, extents are stored in the tree (possibly merged >> with other extents). The ioctl then can walk through the tree, take out >> extents from the tree, compare them with the free extents and possibly >> trim them. >> >> Note that there is support for setting minimum extent length in ioctl >> call, so it will not bother with shorter extents. Also, when the >> previously deleted range is taken from the tree and it is not entirely >> free, the free fragments are discarded and extents shorter than minlen >> are NOT returned back to the tree to avoid fragmentation of the tree >> which could lead to the big memory consumption. >> >> But you may notice, that there is one problem. bb_bitmap_deleted does >> not survive umount. To bypass the problem the first ioctl call have to >> walk through whole file system trimming all free extents. >> >> Signed-off-by: Lukas Czerner >> > > For now it just ignores the small extents to avoid fragmentation. As I > said before, I agree that they should not be ignored, I just need to > figure out the way to do this efficiently. I suggest to not start with optimisations: let's first take a look what is going on in the simple case. Benchmarks are our friends.. Edward. > > > Also it was not properly tested yet. > > > -Lukas. >