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Message-ID: <511A675A.8050004@redhat.com> Date: Tue, 12 Feb 2013 10:01:30 -0600 From: Eric Sandeen <sandeen@...hat.com> To: Adil Mujeeb <mujeeb.adil@...il.com> CC: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org Subject: Re: ext4: Used block count in df On 2/12/13 12:14 AM, Adil Mujeeb wrote: > Hi, > >> My only point is, default ext4 statfs behavior is quite complicated, and it >> looks like you have found a bug related to the calculation of metadata overhead. > > I see. > Where should I report this issue to get it confirm by developers? Here is fine. :) It would be good to file a bug on bugzilla.kernel.org too if you like. The problem is, I think ext4's metadata behavior has gotten so complex, the consensus so far seems to be to just accept the inaccuracy in this style of df reporting: * Note: calculating the overhead so we can be compatible with * historical BSD practice is quite difficult in the face of * clusters/bigalloc. This is because multiple metadata blocks from * different block group can end up in the same allocation cluster. * Calculating the exact overhead in the face of clustered allocation * requires either O(all block bitmaps) in memory or O(number of block * groups**2) in time. We will still calculate the superblock for * older file systems --- and if we come across with a bigalloc file * system with zero in s_overhead_clusters the estimate will be close to * correct ... but it is odd behavior, and filing a bug would probably be good. -Eric >> It should only be a reporting issue, and should not cause any runtime issues. > > OK, I understand. > > Thanks, > Adil > > On Mon, Feb 11, 2013 at 11:02 PM, Eric Sandeen <sandeen@...hat.com> wrote: >> On 2/11/13 12:36 AM, Adil Mujeeb wrote: >>> Thanks Eric. >>> >>>>> 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: >>> >>> its 1.41.12. >>> >>>> 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. >>> >>> I also thinks so. But with this assumption, the number of 1KB blocks >>> used should increase as per filesystem size increase. No? >>> >>>> >>>> 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. >>> >>> Agree if "bsd df" assumes so. >>> >>>> 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." >>> >>> In your example, dumpe2fs and minix df both are reporting same value, isn't it? >>> >>> I am still not able to understand why increasing the filesystem size >>> decreases used 1K block count :( >>> Am I missing some basic things here? Sorry if i am not able to catch >>> your point :( >> >> My only point is, default ext4 statfs behavior is quite complicated, and it >> looks like you have found a bug related to the calculation of metadata overhead. >> >> It should only be a reporting issue, and should not cause any runtime issues. >> >> Thanks, >> -Eric >> >>> Regards, >>> Adil >> >> -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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