Hi Eric, On Tue, Feb 12, 2013 at 9:31 PM, Eric Sandeen <sandeen@xxxxxxxxxx> wrote: > 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. I filed the bug in bugzilla. https://bugzilla.kernel.org/show_bug.cgi?id=53741 Thank for all your inputs :) Regards, Adil > > -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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
