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
