Re: 'btrfs filesystem defragment' makes files explode in size, especially fallocated ones

[Hanabishi <i.r.e.c.c.a.k.u.n+kernel.org@gmail.com>]

On 8/6/24 12:17, Hanabishi wrote:

> In fact fiemap "TOTAL" adds up correctly to the actual file size here.
> So maybe it is actually compsize lying with "Disk Usage" or something else weird happening.

I reproduced the results on a dedicated disk.
No, compsize is not lying. Confirmed by looking at total fs usage.

# compsize mingw-w64-gcc-13.1.0-1-x86_64.pkg.tar.zst
Processed 1 file, 3 regular extents (3 refs), 0 inline.
Type       Perc     Disk Usage   Uncompressed Referenced
TOTAL      100%      449M         449M         224M
none       100%      449M         449M         224M

# btrfs filesystem usage /mnt
Overall:
     Device size:		 29.88GiB
     Device allocated:		  1.52GiB
     Device unallocated:		 28.36GiB
     Device missing:		    0.00B
     Device slack:		    0.00B
     Used:			450.82MiB
     Free (estimated):		 28.92GiB	(min: 14.74GiB)
     Free (statfs, df):		 28.92GiB
     Data ratio:			     1.00
     Metadata ratio:		     2.00
     Global reserve:		  5.50MiB	(used: 16.00KiB)
     Multiple profiles:		       no

Data,single: Size:1.00GiB, Used:449.51MiB (43.90%)
    /dev/sdc1	  1.00GiB

Metadata,DUP: Size:256.00MiB, Used:656.00KiB (0.25%)
    /dev/sdc1	512.00MiB

System,DUP: Size:8.00MiB, Used:16.00KiB (0.20%)
    /dev/sdc1	 16.00MiB

Unallocated:
    /dev/sdc1	 28.36GiB

Notice that the space overhead does *not* belong to metadata. It is the actual data space wasted. So the problem is real.
Which also means that fiemap is the one who lies here.

# xfs_io -c "fiemap -v" mingw-w64-gcc-13.1.0-1-x86_64.pkg.tar.zst
mingw-w64-gcc-13.1.0-1-x86_64.pkg.tar.zst:
  EXT: FILE-OFFSET      BLOCK-RANGE       TOTAL FLAGS
    0: [0..460287]:     7335440..7795727 460288   0x0
    1: [460288..460303]: 7335424..7335439     16   0x1