On 2017-02-08 09:46, Peter Grandi wrote:
My system is or seems to be running out of disk space but I
can't find out how or why. [ ... ]
Filesystem Size Used Avail Use% Mounted on
/dev/sda3 28G 26G 2.1G 93% /
[ ... ]
So from chunk level, your fs is already full. And balance
won't success since there is no unallocated space at all.
To add to this, 28GiB is a bit too small for Btrfs, because at
that point chunk size is 1GiB. I have the habit of sizing
partitions to an exact number of GiB, and that means that most
of 1GiB will never be used by Btrfs because there is a small
amount of space allocated that is smaller than 1GiB and thus
there will be eventually just less than 1GiB unallocated.
Unfortunately the chunk size is not manually settable.
28GB is a perfectly reasonable (if a bit odd) size for a non-mixed-mode
volume. The issue isn't total size, it's the difference between total
size and the amount of data you want to store on it. and how well you
manage chunk usage. If you're balancing regularly to compact chunks
that are less than 50% full, you can get away with as little as 4GB of
extra space beyond your regular data-set with absolutely zero issues.
I've run full Linux installations in VM's with BTRFS on 16GB disk images
before with absolutely zero issues, and have a handful of fairly active
8GB BTRFS volumes on both of my primary systems that never have any
issues with free space despite averaging 5GB of space usage.
Example here from 'btrfs fi usage':
Overall:
Device size: 88.00GiB
Device allocated: 86.06GiB
Device unallocated: 1.94GiB
Device missing: 0.00B
Used: 80.11GiB
Free (estimated): 6.26GiB (min: 5.30GiB)
That means that I should 'btrfs balance' now, because of the
1.94GiB "unallocated", 0.94GiB will never be allocated, and that
leaves just 1GiB "unallocated" which is the minimum for running
'btrfs balance'. I have just done so and this is the result:
Actually, that 0.94GB would be used. BTRFS will create smaller chunks
if it has to, so if you allocated two data chunks with that 1.94GB of
space, you would get one 1GB chunk and one 0.94GB chunk.
Overall:
Device size: 88.00GiB
Device allocated: 82.03GiB
Device unallocated: 5.97GiB
Device missing: 0.00B
Used: 80.11GiB
Free (estimated): 6.26GiB (min: 3.28GiB)
At some point I had decided to use 'mixedbg' allocation to
reduce this problem and hopefully improve locality, but that
means that metadata and data need to have the same profile, and
I really want metadata to be 'dup' because of checksumming,
and I don't want data to be 'dup' too.
You could also use larger partitions and keep a better handle on free space.
[ ... ] To proceed, add a larger device to current fs, and do
a balance or just delete the 28G partition then btrfs will
handle the rest well.
Usually for this I use a USB stick, with a 1-3GiB partition plus
a bit extra because of that extra bit of space.
If you have a lot of RAM and can guarantee that things won't crash (or
don't care about the filesystem too much and are just trying to avoid
having to restore a backup), a ramdisk works well for this too.
https://btrfs.wiki.kernel.org/index.php/FAQ#How_much_free_space_do_I_have.3F
https://btrfs.wiki.kernel.org/index.php/FAQ#Help.21_Btrfs_claims_I.27m_out_of_space.2C_but_it_looks_like_I_should_have_lots_left.21
marc.merlins.org/perso/btrfs/post_2014-05-04_Fixing-Btrfs-Filesystem-Full-Problems.html
Unfortunately if it is a single device volume and metadata is
'dup' to remove the extra temporary device one has first to
convert the metadata to 'single' and then back to 'dup' after
removal.
This shouldn't be needed, if it is then it's a bug that should be
reported and ideally fixed (there was such a bug when converting from
multi-device raid profiles to single device, but that got fixed quite a
few kernel versions ago (I distinctly remember because I wrote the fix)).
There are also some additional reasons why space used (rather
than allocated) may be larger than expected, in special but not
wholly infrequent cases. My impression is that the Btrfs design
trades space for performance and reliability.
In general, yes, but a more accurate statement would be that it offers a
trade-off between space and convenience. If you're not going to take
the time to maintain the filesystem properly, then you will need more
excess space for it.
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