zfs cannot create snapshot, out of space

Question

I have a disk with these layers: sata disk, luks, zpool, ext4

The ext4 fs was created with these commands:

cryptsetup -v luksFormat /dev/sdb
cryptsetup luksOpen /dev/sda store02
zpool create zstore02 /dev/mapper/store02
zfs create -V 1600G zstore02/dsk02
mkfs.ext4 -L dsk02 /dev/zstore02/dsk02

System is Ubuntu 20.04.1 LTS. This pool was later imported on another 20.04.1 system.

I want to create a read/write snapshot and mount it. But it tells me "out of space":

root@computer:~# zpool list
NAME       SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
zstore02  1,81T  1,50T   320G        -         -     7%    82%  1.00x    ONLINE  -
root@computer:~# zfs list
NAME             USED  AVAIL     REFER  MOUNTPOINT
zstore02        1,61T   148G       24K  /zstore02
zstore02/dsk02  1,61T   262G     1,50T  -
root@computer:~# zfs snapshot zstore02/dsk02@test
cannot create snapshot 'zstore02/dsk02@test': out of space
root@computer:~# zfs get reservation zstore02/dsk02
NAME            PROPERTY     VALUE   SOURCE
zstore02/dsk02  reservation  none    local
root@computer:~# zfs set reservation=10G zstore02
root@computer:~# zfs set reservation=10G zstore02/dsk02
root@computer:~# zfs snapshot zstore02/dsk02@test
cannot create snapshot 'zstore02/dsk02@test': out of space
root@computer:~#

It does not work with reserved=none nor reserved=10G.

Apparently, "zpool list" reports 300G free space, and "zfs list" reports 262G free space. But somehow I cannot create a snapshot. Why?

UPDATE

If I try to create a new volume, then it works (sorry for the Hungarian locale):

root@computer:~# zfs create -V 1G zstore02/test
root@computer:~# mkfs.ext4 -L test /dev/zstore02/test
mke2fs 1.45.5 (07-Jan-2020)
Eszközblokkok eldobása: kész
Fájlrendszer létrehozása 262144 4 blokkal és 65536 inode-dal
Fájlrendszer UUID: 14e07b33-5d25-465a-aeb8-7fbfe2499dfd
Tartalék szuperblokkok tárolva a blokkokon:
        32768, 98304, 163840, 229376

Csoporttáblák foglalása: kész
Inode táblák írásakor: kész
Napló létrehozása (8192 blokk): kész
Szuperblokkok és fájlrendszer-könyvelési információk írása: kész

root@computer:~# zfs snapshot zstore02/test@snap1

root@computer:~# zfs list -t snapshot
NAME                  USED  AVAIL     REFER  MOUNTPOINT
zstore02/test@snap1     0B      -     32,7M  -
root@computer:~#

It works for any volume that I create, except zstore02/dsk02.

But why?

Answer 1

You have a refreservation set on your zvol, which reserves disk space for that dataset or zvol without regard to snapshots or clones. In order to make a snapshot there must be enough unreserved space left in the dataset to accommodate its referenced space, in this case 1.50T. Since your refreservation is (most likely) 1.61T you basically cannot do much of anything as all the free space gets reserved for that zvol (which will never use it).

To fix the problem, get rid of the refreservation.

zfs set refreservation=none zstore02/dsk02

To avoid the problem in future, create zvols with the -s (sparse) flag, which will also thin provision them:

zfs create -s -V 1600G zstore02/dsk02

If a thick provisioned volume is desired instead, create the volume and then set refreservation=auto.

zfs set refreservation=auto zstore02/dsk02