On Mon 03-09-18 17:20:21, Liu Bo wrote: > On Mon, Sep 3, 2018 at 2:53 AM, Jan Kara <jack@xxxxxxx> wrote: > > On Sun 02-09-18 23:58:56, Liu Bo wrote: > >> My question is, > >> Is there a way to calculate how much space metadata has occupied? > >> > >> So the case I've run into is that 'df /mnt' shows > >> > >> Filesystem 1K-blocks Used Available Use% Mounted on > >> /dev/sdc 882019232 26517428 811356348 4% /mnt > >> > >> but 'du -s /mnt' shows > >> 13347132 /mnt > >> > >> And this is a freshly mounted ext4, so no deleted files or dirty data exist. > >> > >> The kernel is quite old (2.6.32), but I was just wondering, could it > >> be due to metadata using about 13G given the whole filesystem is 842G? > > > > Yes, that sounds plausible. > > > >> I think it has nothing to do with "Reserved block counts" as df > >> calculates "Used" in ext4_statfs() by "buf->f_blocks - buf->f_bfree". > >> > >> So if there is a way to know the usage of metadata space, via either > >> manual analysis from the output of dumpe2fs/debugfs or a tool, could > >> you please suggest? > > > > So journal takes up some space. Debugfs command: > > > > stat <8> > > > > Inode table takes lots of blocks: > > > > stats > > > > search for "Inode count", multiply by "Inode size". Then there are bitmap > > blocks - count 2 blocks for each group. The rest of overhead should be > > pretty minimal. > > > > Thank you so much for the reply, Jan. > > Per what you've mentioned, the journal + inode table have taken >14G > in this ext4, so that's a lot of space, good. > > And I digged it further, I found that the overhead from (journal + > inode_table + block_bitmap) has been excluded in the output of 'df' as > ext4_statfs() gets buf->f_blocks by > > buf->f_blocks = ext4_blocks_count(es) - EXT4_C2B(sbi, overhead); > > > and ->f_blocks is shown as "Total", but there is still some gap > between "Used" in df (26517428 * 1024) and the summary report of "du > -s" (13347132 * 1024), Correct, I forgot about this. > -------- > # df /mnt > Filesystem 1K-blocks Used Available Use% Mounted on > /dev/sdc 882019232 26517428 811356348 4% /mnt > > #du -s /mnt > 13347132 /mnt > -------- > > Now I'm even more curious, any idea where could those gap come from? Interesting question. So e.g. for my test filesystem, the gap is caused by 'resize_inode' (inode number 7), which is reserving blocks at the beginning of block groups to allow for growing of group descriptors. So check whether you have resize_inode feature enabled (stats command in debugfs). Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
