From: Eric Sandeen Subject: Re: ext4: Used block count in df Date: Thu, 07 Feb 2013 10:49:49 -0600 Message-ID: <5113DB2D.4000305@redhat.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Adil Mujeeb Return-path: Received: from mx1.redhat.com ([209.132.183.28]:8826 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758908Ab3BGQtz (ORCPT ); Thu, 7 Feb 2013 11:49:55 -0500 In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On 2/7/13 12:39 AM, Adil Mujeeb wrote: > Hi, > > I have an observation on EXT4 filesystem. I created filesystem of size > 1TB, 4TB, and 7TB and then checked the output of df command. Telling us which version of e2fsprogs and which kernel would be helpful, but: > df command showed the number of 1KB blocks used. The result was: > 1TB: 204056 > 4TB: 198680 > 7TB: 181784 extN makes df complicated in several ways. It reserves blocks for the superuser (5% by default) and also uses a lot of blocks up-front for filesytem metadata - inode tables, block bitmaps, and the like. But what you are seeing here is this: It also defaults to "bsd df" which does not count filesystem metadata when telling you about the number of blocks used. So in theory, a freshly made fs should actually tell you 0 blocks used, I think. Looking at the dumpe2fs output for the 4t file, I see: # dumpe2fs -h 4tfile-ext4 | grep -i block dumpe2fs 1.41.12 (17-May-2010) Block count: 1073741824 Reserved block count: 53687091 Free blocks: 1056843748 ... and 1073741824-1056843748 is 16898076 4k blocks, or 67592304 1k blocks actually used. If we ask for "minix df" by mounting with -o minixdf which is true blocks used, we get: # df 4t-ext4/ Filesystem 1K-blocks Used Available Use% Mounted on /mnt/test2/mkfs-test/4tfile-ext4 4294967296 67592304 4012626628 2% /mnt/test2/mkfs-test/4t-ext4 I'd say this appears to be a slight inaccuracy in ext4_statfs, coupled with the strangeness of the "bsd df" reporting. It is apparently miscalculating the filesystem metadata "overhead." > I performed the same on XFS and the result was: > 1TB: 32928 > 4TB: 32928 > 7TB: 33024 XFS is straightforward; blocks used for metadata count as "used." Every other block is free and available. No fiddling around, just like with the minixdf mount option for extN. -Eric > EXT4 result shows with increasing filesystem size, the number of used > blocks decreased. I dont have idea about low level implementation but > I am curious why it is so? > > Thanks. > > Regards, > Adil > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >