On Wed, Feb 08, 2017 at 02:46:32PM +0000, 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.
Not true -- the last chunk can be smaller than 1 GiB, to use the available space completely. Hugo. > Unfortunately the chunk size is not manually settable. > > 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: > > 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. > > > [ ... ] 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. > > 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. > > 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. -- Hugo Mills | Alert status chocolate viridian: Authorised hugo@... carfax.org.uk | personnel only. Dogs must be carried on escalator. http://carfax.org.uk/ | PGP: E2AB1DE4 |
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