Fajar A. Nugraha wrote:
> On Wed, Dec 28, 2011 at 11:57 PM, Martin Steigerwald
> <mar...@lichtvoll.de> wrote:
>> But BTRFS does not:
>>
>> merkaba:~> fstrim -v /
>> /: 4431613952 bytes were trimmed
>> merkaba:~> fstrim -v /
>> /: 4341846016 bytes were trimmed
>
> .... and apparently it can't trim everything. Or maybe my kernel is
> just too old.
>
>
> $ sudo fstrim -v /
> 2258165760 Bytes was trimmed
>
> $ df -h /
> Filesystem Size Used Avail Use% Mounted on
> /dev/sda6 50G 34G 12G 75% /
>
> $ mount | grep "/ "
> /dev/sda6 on / type btrfs (rw,noatime,subvolid=258,compress-force=lzo)
>
> so only about 2G out of 12G can be trimmed. This is on kernel 3.1.4.
>
That's because only free spaces in block groups will be trimmed. Btrfs
allocates space from block groups, and when there's no space availabe,
it will allocate a new block group from the pool. In your case there's
~10G in the pool.
You can do a "btrfs fi df /", and you'll see the total size of existing
block groups.
You can empty the pool by:
# dd if=/dev/zero of=/mytmpfile bs=1M
Then release the space (but it won't return back to the pool):
# rm /mytmpfile
# sync
and try "btrfs fi df /" and trim again.
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