Re: understanding disk space usage

[Hugo Mills <hugo@carfax.org.uk>]

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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
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