I recently started using btrfs, in both raid0 and raid1 setups, and migrated some data there.

Once all the data was copied, I wanted to check how much overhead moving to btrfs meant, space-wise.

# df -h /mnt/share
Filesystem Size Used Avail Use% Mounted on
/dev/sdb3 932G 682G 250G 74% /mnt/share

# btrfs-show /dev/sdb3
Label: none uuid: e06484de-fb18-4564-b6e6-adbaed8bebec
Total devices 2 FS bytes used 681.61GB
devid 2 size 465.66GB used 427.13GB path /dev/sdb3
devid 1 size 465.66GB used 427.15GB path /dev/sda3


The filesystem was freshly created, in a raid0 fashion, so both 465GB partitions are used to form (roughly) a 930GB volume.
So, I had filled it with 682GB of data, and df was telling me 250GB were left, which is not unexpected. What I didn't expect, though, is btrfs-show telling me 427GB were used on each partition. Again, the filesystem was freshly created and the data had just been copied over, so I wasn't expecting these numbers to be off.

Which meant 854GB were being allocated on disk for 681GB of data, 25% overhead !

Curious to know how much left that really meant, I just tried a stupid thing: filling as much as btrfs permits:

# dd if=/dev/zero of=/mnt/share/zero bs=1M
dd: writing `/mnt/share/zero': No space left on device
253359+0 records in
253358+0 records out
265665118208 bytes (266 GB) copied, 1144.41 s, 232 MB/s
# du -sh /mnt/share/zero
248G /mnt/share/zero


It turns out I actually could write for the amount of free space df was claiming was free (modulo GiB-GB conversions).

# df -h /mnt/share
Filesystem Size Used Avail Use% Mounted on
/dev/sdb3 932G 930G 2.0G 100% /mnt/share

# btrfs-show /dev/sdb3
Label: none uuid: e06484de-fb18-4564-b6e6-adbaed8bebec
Total devices 2 FS bytes used 929.36GB
devid 2 size 465.66GB used 465.13GB path /dev/sdb3
devid 1 size 465.66GB used 465.15GB path /dev/sda3


So, this time df is slightly off, and btrfs-show somehow agrees the filesystem is full.

But surprises don't stop there:

# rm /mnt/share/zero

# df -h /mnt/share
Filesystem Size Used Avail Use% Mounted on
/dev/sdb3 932G 682G 250G 74% /mnt/share

# btrfs-show /dev/sda3
Label: none uuid: e06484de-fb18-4564-b6e6-adbaed8bebec
Total devices 2 FS bytes used 929.36GB
devid 2 size 465.66GB used 465.13GB path /dev/sdb3
devid 1 size 465.66GB used 465.15GB path /dev/sda3


That's right, even after the file removal, for btrfs-show, everything is still in use. Even after several sync or several hours later.

All in all, just don't trust what btrfs-show says.

NB: one also needs to know that df reports wrong sizes for raid1 setups, but that's another story.

2010-09-25 17:53:53+0900

p.d.o

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