On Tue, Sep 4, 2018 at 12:52 AM, Jan Kara <jack@xxxxxxx> wrote: > 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). You're right, "resize_inode" is enabled, and I found on 4.18 at least two things take some space in an empty ext4, i.e. 'resize_inode' and reserved blocks for metadata(2% or 16K). thanks, liubo
