Cannot compact dynamic VDI with FreeBSD on ZFS

[TCH]

I have a VBox VDI with FreeBSD on ZFS. The VDI is 22.8 GB, while everything in FreeBSD reports, that only around 9 GB is used. Maxnix helped me to set the initialization value to zero for 'zpool initialize', but even after that, 'VBoxManage modifymedium --compact' does not compact the image.

Output of 'df -h /':

Filesystem            Size    Used   Avail Capacity  Mounted on
zroot/ROOT/default     36G    8.6G     28G    24%    /

Output of 'zfs list':

NAME                 USED  AVAIL     REFER  MOUNTPOINT
zroot               9.24G  27.6G       88K  /zroot
zroot/ROOT          8.56G  27.6G       88K  none
zroot/ROOT/default  8.56G  27.6G     8.56G  /
zroot/tmp            452K  27.6G      452K  /tmp
zroot/usr            686M  27.6G       88K  /usr
zroot/usr/home        88K  27.6G       88K  /usr/home
zroot/usr/ports      686M  27.6G      686M  /usr/ports
zroot/usr/src         88K  27.6G       88K  /usr/src
zroot/var            716K  27.6G       88K  /var
zroot/var/audit       88K  27.6G       88K  /var/audit
zroot/var/crash       88K  27.6G       88K  /var/crash
zroot/var/log        252K  27.6G      252K  /var/log
zroot/var/mail       112K  27.6G      112K  /var/mail
zroot/var/tmp         88K  27.6G       88K  /var/tmp

Output of 'zpool status -x -v':

  pool: zroot
 state: ONLINE
status: One or more devices has experienced an error resulting in data
	corruption.  Applications may be affected.
action: Restore the file in question if possible.  Otherwise restore the
	entire pool from backup.
   see: https://openzfs.github.io/openzfs-docs/msg/ZFS-8000-8A
config:

	NAME        STATE     READ WRITE CKSUM
	zroot       ONLINE       0     0     0
	  ada0p3    ONLINE       0     0     0

errors: Permanent errors have been detected in the following files:

        zroot/ROOT/default:<0x6814b>

(The corrupted file in question was '/lib/libzpool.so.2', but i've downloaded the FreeBSD 13.4 DVD image and restored the file from there. It no longer reports the file being corrupted, but the ZFS error remained. I do not know, if this is related to the compacting problem, or not.)

There are no snapshots in the system, they were destroyed via:

zfs list -H -o name -t snapshot | xargs -n1 zfs destroy -R

Can anyone give me some hints about what can be the problem?