I'm a bit puzzled about the way ext4 reports used space. On a new Debian wheezy (testing) installation, I consistently got about 1GB extra space used with a 60GB SSD, when compared to du. I then network booted that computer and mounted the SSD. The network booted OS (Debian squeeze) shows only 180MB extra space used on the SSD. The journal is 128MB, so there's not much extra beyond that anymore.
What is the extra used space reported by df, when it's not space reserved for root or deleted files? Using Google mostly results in those usual causes, which I have ruled out. And why does that amount differ on the same filesystem when mounted on different operating systems? I tested this by re-installing, and the extra usage was again ~1GB on the wheezy installation, and 180MB when I network booted and mounted that SSD.
On XFS and btrfs, the extra reported usage seems negligible. I realize that the filesystem needs some overhead, but it's inconvenient when that overhead is mixed up with the real usage.
Here are some detailed outputs.
df
$ df -m on wheezy
Filesystem 1M-blocks Used Available Use% Mounted on
rootfs 57132 1567 52703 3% /
udev 7899 0 7899 0% /dev
tmpfs 1581 1 1581 1% /run
/dev/mapper/ssd-kvmhost 57132 1567 52703 3% /
tmpfs 5 0 5 0% /run/lock
tmpfs 3162 0 3162 0% /tmp
tmpfs 3162 0 3162 0% /run/shm
du
$ du -sm /
592 /
tune2fs shows Inode count to be 3670016 and size 256, which indeed explains almost all of the used space. I just don't understand why this isn't subtracted from Size, since the inodes are statically reserved. Counting it as space and then always counting it as used doesn't make much sense.
Here's the output for the very same filesystem, from the network booted Debian squeeze:
aufs 7905 46 7859 1% /
tmpfs 7905 0 7905 0% /lib/init/rw
udev 7900 1 7900 1% /dev
tmpfs 7905 1 7905 1% /dev/shm
172.17.172.127:/storage/private/tftp/debian-live-6.0.3-amd64-lxde-desktop
24737 17448 7289 71% /live/image
tmpfs 7905 46 7859 1% /live/cow
tmpfs 7905 0 7905 0% /live
tmpfs 7905 1 7905 1% /tmp
/dev/mapper/ssd-kvmhost
56337 772 52703 2% /mnt
And just for confirmation, du -sm / on the network booted OS:
592 /
Maybe the older kernel wisely doesn't count the statically reserved inode space as usable, and thus doesn't have to show it as used? The difference is 795MB in both Size and Used, almost all of the space needed for the inodes. What's the 180MB then? If that is also static, wouldn't it be optimal to also subtract that from Size? Then df could actually show real usage, like other filesystems seem to do.
The way I see it, if my filesystem needs a static amount of overhead, then it just provides less usable space than some other filesystems for the same amount of absolute space. Shouldn't df reflect that, and also show how much of the usable space I have used?
Best Answer
ext[234] by default reserves 8192 inodes per 128 mb block group, which takes 2mb, per group, which works out to close to 1gb for a 60 gb filesystem. There should be no difference when you mount the drive in another system. It looks like they may have changed the way the kernel reports used space between wheezy and squeeze, though I have not yet found a commit indicating this was done on purpose.