From: Ted Ts'o Subject: Re: getdents - ext4 vs btrfs performance Date: Wed, 14 Mar 2012 12:48:04 -0400 Message-ID: <20120314164804.GA28042@thunk.org> References: <20120310044804.GB5652@thunk.org> <4F5F9A97.5060404@ubuntu.com> <20120313195339.GA24124@thunk.org> <4F5FAC9C.9070607@gmail.com> <20120313213304.GB11969@thunk.org> <20120314025108.GF15379@thunk.org> <4F60A881.3070607@zabbo.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Yongqiang Yang , Phillip Susi , Andreas Dilger , Lukas Czerner , Jacek Luczak , "linux-ext4@vger.kernel.org" , linux-fsdevel , LKML , "linux-btrfs@vger.kernel.org" To: Zach Brown Return-path: Received: from li9-11.members.linode.com ([67.18.176.11]:47127 "EHLO test.thunk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756141Ab2CNQsK (ORCPT ); Wed, 14 Mar 2012 12:48:10 -0400 Content-Disposition: inline In-Reply-To: <4F60A881.3070607@zabbo.net> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, Mar 14, 2012 at 10:17:37AM -0400, Zach Brown wrote: > > >We could do this if we have two b-trees, one indexed by filename and > >one indexed by inode number, which is what JFS (and I believe btrfs) > >does. > > Typically the inode number of the destination inode isn't used to index > entries for a readdir tree because of (wait for it) hard links. You end > up right back where you started with multiple entries per key. Well, if you are using 32-bit (or even 48-bit) inode numbers and a 64-bit telldir cookie, it's possible to make the right thing happen. But yes, if you are using 32-bit inode numbers and a 32-bit telldir cookie, dealing with what happens when you have multiple hard links to the same inode in the same directory gets tricky. > A painful solution is to have the key in the readdir tree allocated by > the tree itself -- count key populations in subtrees per child pointer > and use that to find free keys. One thing that might work is to have a 16-bit extra field in the directory entry that gives an signed offset to the inode number so that such that inode+offset is a unique value within the btree sorted by inode+offset number. Since this tree is only used for returning entries in an optimal (or as close to optimal as can be arranged) order, we could get away with that. - Ted