From: Valerie Clement Subject: problem with delayed allocation option Date: Fri, 26 Oct 2007 14:28:36 +0200 Message-ID: <4721DD74.20806@bull.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Alex Tomas , Andreas Dilger To: ext4 development Return-path: Received: from ecfrec.frec.bull.fr ([129.183.4.8]:46787 "EHLO ecfrec.frec.bull.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751890AbXJZM3G (ORCPT ); Fri, 26 Oct 2007 08:29:06 -0400 Sender: linux-ext4-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org Hi all, I ran a small test which creates one directory and 2O 8-KB size files i= n it. When the filesystem is mounted without the delalloc option, here is the output of the command dumpe2fs for the group in which the directory and= =20 the files are created: Group 532 : (Blocks 17432576-17465343) Block bitmap at 17432576 (+0), Inode bitmap at 17432577 (+1) Inode table at 17432578-17433089 (+2) 32213 free blocks, 16363 free inodes, 1 directories Free blocks : 17433090-17459199, 17459241-17465343 Free inodes : 8716310-8732672 When the filesystem is mounted with the delalloc option, the same test gives a different result: Group 395 : (Blocks 12943360-12976127) Block bitmap at 12943360 (+0), Inode bitmap at 12943361 (+1) Inode table at 12943362-12943873 (+2) 32213 free blocks, 16363 free inodes, 1 directories Free blocks : 12943874-12955647, 12955650-12955655,=20 12955658-12955663, 12955666-12955671, 12955674-12955679,=20 12955682-12955687, 12955690-12955695, 12955698-12955703,=20 12955706-12955711, 12955714-12955719, 12955722-12955727,=20 12955730-12955735, 12955738-12955743, 12955746-12955751,=20 12955754-12955759, 12955762-12955767, 12955770-12955775,=20 12955778-12955783, 12955786-12955791, 12955794-12955799,=20 12955802-12961791, 12961793-12976127 Free inodes : 6471702-6488064 In the first case, the allocated blocks are contiguous whereas they are not in the second case. After adding traces in the code to understand why the behavior is different with the delalloc option, I found that the problem is related to the inode reservation window. To simplify, without the delalloc option we have the following scenario= : =46or each inode, - call alloc_new_reservation() to allocate a new reservation window - allocate blocks for data - write data to disk - ext4_discard_reservation() when the inode is closed. With the delalloc option, when the data are written to disk we have: =46or each inode, - call alloc_new_reservation() to allocate a new reservation window - allocate blocks for data - write data to disk I think a call to ext4_discard_reservation() is missing somewhere and the question is where. I tried to add this call at the end of the ext4_da_get_block_write() function. This seems to fix the problem as the blocks are allocated contiguously on disk but the function seems to be called too many times so I think it is perhaps not the right place to call it. Who could look into this problem? I've got a few days off so I couldn't help more next days, but the problem is easily reproductible. Wouldn't this also explain why the compilebench results posted by Chris Mason are bad in some cases? Val=E9rie