From: Martin Steigerwald Subject: Re: Large buffer cache in EXT4 Date: Sun, 17 Feb 2013 11:19:11 +0100 Message-ID: <201302171119.11371.Martin@lichtvoll.de> References: <6F61E227-A6F7-4AD6-9B9C-39E9E9EAABFA@dilger.ca> (sfid-20130217_102626_808359_E4FE7377) Mime-Version: 1.0 Content-Type: Text/Plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Subranshu Patel , "linux-ext4@vger.kernel.org" To: Andreas Dilger Return-path: Received: from mondschein.lichtvoll.de ([194.150.191.11]:50288 "EHLO mail.lichtvoll.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755934Ab3BQKTN convert rfc822-to-8bit (ORCPT ); Sun, 17 Feb 2013 05:19:13 -0500 In-Reply-To: <6F61E227-A6F7-4AD6-9B9C-39E9E9EAABFA@dilger.ca> Sender: linux-ext4-owner@vger.kernel.org List-ID: Am Sonntag, 17. Februar 2013 schrieb Andreas Dilger: > On 2013-02-16, at 21:04, Subranshu Patel wrote: > > I created 2 filesystem on my system (RHEL 6.3, kernel version 2.6.3= 2) > > - XFS and EXT4 and mounted them. > >=20 > > On both the filesystem I executed the mdtest tool(opensource tool) = for > > 64 concurrent process. Each process performed the following: > > - Create large number of directories > > - Remove all the directories > >=20 > > During this time I monitored the memory usage of the system using s= ar > > command. I checked the 3 components - kbmemused, kbbuffers and > > kbcached > >=20 > > kbmemused - Amount of used memory in kilobytes. This does not take > > into account memory used by the kernel itself. > > kbbuffers - buffer cache > > kbcached - page cache > >=20 > > While the kbmemused and kbcached component was almost similar in EX= T4 > > and XFS (XFS being a little higher), the kbbuffer showed a totally > > different trend. > >=20 > > For EXT4, kbbuffers was: > > 390999KB for dir creation > > 364803KB for dir removal > > For XFS, kbbuffers was: > >=20 > > 1701KB for dir creation > > 2738KB for dir removal > >=20 > > In kernel 2.6, both buffer cache and page cache are merged. The pag= e > > cache caches pages of files. The buffer cache caches disk blocks wh= ich > > consists of mainly metadata (not file data). > >=20 > > Why is the buffer cache large in case of EXT4 and what is stored in > > the buffer cache? >=20 > XFS does not use buffer cache, while ext[234] does use buffer cache. >=20 > This is just a different code design. Ext4 uses the buffer cache to t= rack > metadata for journaling. Doesn=B4t XFS use its own mechanism with xfsbufd kernel thread? Thanks, --=20 Martin 'Helios' Steigerwald - http://www.Lichtvoll.de GPG: 03B0 0D6C 0040 0710 4AFA B82F 991B EAAC A599 84C7 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html