From: Frank Mayhar Subject: Re: Question on fallocate/ftruncate sequence Date: Thu, 23 Jul 2009 09:27:02 -0700 Message-ID: <1248366422.27509.1.camel@bobble.smo.corp.google.com> References: <6601abe90907200936w61ebda92reae368a2b9efac66@mail.gmail.com> <4A64F37D.7020803@redhat.com> <1248211771.20743.2.camel@bobble.smo.corp.google.com> <20090721215421.GM4231@webber.adilger.int> <1248304214.14463.17.camel@bobble.smo.corp.google.com> <4A67D36D.20708@redhat.com> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: Andreas Dilger , Curt Wohlgemuth , ext4 development To: Eric Sandeen Return-path: Received: from smtp-out.google.com ([216.239.33.17]:58271 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753709AbZGWQ2Q (ORCPT ); Thu, 23 Jul 2009 12:28:16 -0400 In-Reply-To: <4A67D36D.20708@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, 2009-07-22 at 22:05 -0500, Eric Sandeen wrote: > Frank Mayhar wrote: > > On Tue, 2009-07-21 at 15:54 -0600, Andreas Dilger wrote: > > ... > > >> That said, we might need to have some kind of flag in the on-disk > >> inode to indicate that it was preallocated beyond EOF. Otherwise, > >> e2fsck will try and extend the file size to match the block count, > >> which isn't correct. We could also use this flag to determine if > >> truncate needs to be run on the inode even if the new size is the > >> same. > > > > After chatting with Curt about this today, it sounds like this needs two > > things. One is your flag in the on-disk inode, set in fallocate() to > > indicate that it has an allocation past EOF. E2fsck would use this to > > avoid "fixing" the file size to match the block count. Truncate would > > use this to notice that there are blocks allocated past i_size and get > > rid of them. It would be cleared by truncate or by ext4_get_blocks when > > using the last block of such an allocation. > > > > Does this make sense? Have I missed anything? > > I guess I'm not totally sold on the new on-disk flag; we can work out > blocks past EOF w/o needing a new flag can't we? It's on-disk because e2fsck needs it to know when not to extend i_size to the actual allocated length of the file. Were it not for that we could easily solve the fallocate/trucate problem with an in-memory flag only. -- Frank Mayhar Google, Inc.