fstrim problem/bug

fstrim problem/bug

[M G Berberich]

Hello,

On a filesystem with 40 G free space and 54 G used, ‘fstrim -v’ gave
this result:

    # fstrim -v /
    /: 0 B (0 bytes) trimmed

After running balance it gave a more sensible

    # fstrim -v /
    /: 37.3 GiB (40007368704 bytes) trimmed

As far as I understand, fstrim should report any unused block to the
disk, so its controller can reuse these blocks. I expected ’fstrim -v’
to report about 40 G trimmed. The fact, that after balance fstrim
reports a sensible amount of trimmed bytes leads to the conclusion,
that fstrim on btrfs does not report unused blocks to the disk (as it
should), but only the blocks of unused chunks. As the fstrim-command
only does a ‘ioctl(fd, FITRIM, &range))’ this seems to be a bug in the
fstrim kernel-code.
In the field this means, that without regularly running balance,
fstrim does not work on btrfs.

	MfG
	bmg

-- 
„Des is völlig wurscht, was heut beschlos- | M G Berberich
 sen wird: I bin sowieso dagegn!“          | mail@m-berberich.de
(SPD-Stadtrat Kurt Schindler; Regensburg)  | 

Re: fstrim problem/bug

[Henk Slager]

On Thu, Jul 7, 2016 at 11:46 AM, M G Berberich <btrfs@oss.m-berberich.de> wrote:
> Hello,
>
> On a filesystem with 40 G free space and 54 G used, ‘fstrim -v’ gave
> this result:
>
>     # fstrim -v /
>     /: 0 B (0 bytes) trimmed
>
> After running balance it gave a more sensible
>
>     # fstrim -v /
>     /: 37.3 GiB (40007368704 bytes) trimmed
>
> As far as I understand, fstrim should report any unused block to the
> disk, so its controller can reuse these blocks. I expected ’fstrim -v’
> to report about 40 G trimmed. The fact, that after balance fstrim
> reports a sensible amount of trimmed bytes leads to the conclusion,
> that fstrim on btrfs does not report unused blocks to the disk (as it
> should), but only the blocks of unused chunks. As the fstrim-command
> only does a ‘ioctl(fd, FITRIM, &range))’ this seems to be a bug in the
> fstrim kernel-code.
> In the field this means, that without regularly running balance,
> fstrim does not work on btrfs.

hmm, yes indeed I see this as well:

# btrfs fi us /
Overall:
   Device size:                  55.00GiB
   Device allocated:             46.55GiB
   Device unallocated:            8.45GiB
   Device missing:                  0.00B
   Used:                         39.64GiB
   Free (estimated):             13.96GiB      (min: 13.96GiB)
   Data ratio:                       1.00
   Metadata ratio:                   1.00
   Global reserve:              480.00MiB      (used: 0.00B)

Data,single: Size:43.77GiB, Used:38.25GiB
  /dev/sda1      43.77GiB

Metadata,single: Size:2.75GiB, Used:1.39GiB
  /dev/sda1       2.75GiB

System,single: Size:32.00MiB, Used:16.00KiB
  /dev/sda1      32.00MiB

Unallocated:
  /dev/sda1       8.45GiB
# fstrim -v /
/: 9,3 GiB (10014126080 bytes) trimmed
# fallocate -l 5G testfile
# fstrim -v /
/: 4,3 GiB (4644130816 bytes) trimmed

Where the difference between 8.45GiB and 9,3 GiB comes from, I
currently don't understand.