On Mon, Sep 3, 2018 at 2:53 AM, Jan Kara <jack@xxxxxxx> wrote: > Hi, > > 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), -------- # 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? thanks, liubo
