From: Theodore Tso Subject: Re: [PATCH -V2 4/5] ext4: Use both hi and lo bits of the group desc values. Date: Sun, 23 Nov 2008 21:13:02 -0500 Message-ID: <20081124021302.GB2163@mit.edu> References: <1227285875-18011-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <1227285875-18011-2-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <1227285875-18011-3-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <1227285875-18011-4-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <4926F00D.6030403@redhat.com> <20081121174110.GH11212@skywalker> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Eric Sandeen , Coly Li , cmm@us.ibm.com, linux-ext4@vger.kernel.org To: "Aneesh Kumar K.V" Return-path: Received: from BISCAYNE-ONE-STATION.MIT.EDU ([18.7.7.80]:46235 "EHLO biscayne-one-station.mit.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751310AbYKXCPd (ORCPT ); Sun, 23 Nov 2008 21:15:33 -0500 Content-Disposition: inline In-Reply-To: <20081121174110.GH11212@skywalker> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Fri, Nov 21, 2008 at 11:11:10PM +0530, Aneesh Kumar K.V wrote: > On Fri, Nov 21, 2008 at 11:29:49AM -0600, Eric Sandeen wrote: > > Aneesh Kumar K.V wrote: > > > Rename the lower bits with suffix _lo and add helper > > > to access the values. Also rename bg_itable_unused_hi > > > to bg_pad as in e2fsprogs. bg_itable_unused_hi is never > > > used before > > > > Maybe you can add info about under which circumstances (what sort of > > fielsystem geometry) the hi bits are needed? And the failure mode when > > we don't use them, if that's known? > > Coly actually said it was added for inode reservation. But he agreed to > drop the high bits now and add them later once the performance issue > with the patch is addressed. The bg_itable_unused field is used for the lazy initialization of inodes. If the units of this field was blocks was blocks, we wouldn't need bg_itable_unused_hi for quite a larger set of blocks. I don't think we should nuke bg_itable_unused_hi, especially given that we have a patch to support the kernel initializing the inodes (yet to be reviewed). As near as I can tell at the time when the fields were added to e2fsprogs, we hadn't yet assigned the pad field to bg_itable_unused_hi, but it's clearly needed if we want to support a larger number of blocks or inodes per block group. Instead we should create the helper functions for bg_itable_unused, and update e2fsprogs to also support these fields. Adding e2fsprogs for large block groups is lower priority than large block numbers, though. - Ted