Remove ubuntu zfs snapshots

There are so many snapshots when using zfs in ubuntu.

Issue

When tried to do release update, got following error

# do-release-update
...
...
Not enough free disk space 

The upgrade has aborted. The upgrade needs a total of 256 M free 
space on disk '/boot'. Please free at least an additional 91.4 M of 
disk space on '/boot'. You can remove old kernels using 'sudo apt 
autoremove' and you could also set COMPRESS=xz in 
/etc/initramfs-tools/initramfs.conf to reduce the size of your 
initramfs. 
...

This error messsage was occurred many times before, but those systems had very small /boot partition or many old kernels kept. If it is the first case, total repartitioning and moving root filesystem are required.

Space on /boot

Examing disk space for bpool, found that zfs reported 675MB used in bpool, but actual usage is only 242MB.

root@ubuntu:~# zpool list
NAME    SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
bpool   960M   675M   285M        -         -    30%    70%  1.00x    ONLINE  -
rpool  17.5G  7.99G  9.51G        -         -    21%    45%  1.00x    ONLINE  -
root@ubuntu:~# zfs list bpool
NAME    USED  AVAIL     REFER  MOUNTPOINT
bpool   675M   157M       96K  /boot
root@ubuntu:~# du -cshx /boot
242M    /boot
242M    total
root@ubuntu:~# 

Then found many snapshots both in bpool and data pool

root@ubuntu:~# zfs list -t snapshot | head
NAME                                                               USED  AVAIL     REFER  MOUNTPOINT
bpool/BOOT/ubuntu_e8m8h0@autozsys_ywm1ok                             0B      -      238M  -
bpool/BOOT/ubuntu_e8m8h0@autozsys_ms74md                             0B      -      238M  -
bpool/BOOT/ubuntu_e8m8h0@autozsys_ugu9z7                            80K      -      242M  -
bpool/BOOT/ubuntu_e8m8h0@autozsys_r3xqau                            72K      -      242M  -
bpool/BOOT/ubuntu_e8m8h0@autozsys_nkagbh                             0B      -      242M  -
bpool/BOOT/ubuntu_e8m8h0@autozsys_xdbwsy                             0B      -      242M  -
bpool/BOOT/ubuntu_e8m8h0@autozsys_zrt7vi                            72K      -      242M  -
bpool/BOOT/ubuntu_e8m8h0@autozsys_jbmnwk                            72K      -      242M  -
bpool/BOOT/ubuntu_e8m8h0@autozsys_0e5p2e                            64K      -      242M  -
root@ubuntu:~# 
root@ubuntu:~# zfs list -t snapshot | wc
    301    1505   27701

Too many! Not sure how many snapshots ubuntu likes to create

Removing snapshots

List all snapshots for /boot

root@ubuntu:~# df /boot
Filesystem               1K-blocks   Used Available Use% Mounted on
bpool/BOOT/ubuntu_e8m8h0    408192 247808    160384  61% /boot
root@ubuntu:~# zfs list -H -o name -t snapshot bpool/BOOT/ubuntu_e8m8h0
bpool/BOOT/ubuntu_e8m8h0@autozsys_ywm1ok
bpool/BOOT/ubuntu_e8m8h0@autozsys_ms74md
bpool/BOOT/ubuntu_e8m8h0@autozsys_ugu9z7
bpool/BOOT/ubuntu_e8m8h0@autozsys_r3xqau
bpool/BOOT/ubuntu_e8m8h0@autozsys_nkagbh
bpool/BOOT/ubuntu_e8m8h0@autozsys_xdbwsy
bpool/BOOT/ubuntu_e8m8h0@autozsys_zrt7vi
bpool/BOOT/ubuntu_e8m8h0@autozsys_jbmnwk
bpool/BOOT/ubuntu_e8m8h0@autozsys_0e5p2e
bpool/BOOT/ubuntu_e8m8h0@autozsys_b17dwn
bpool/BOOT/ubuntu_e8m8h0@autozsys_uad1rb
bpool/BOOT/ubuntu_e8m8h0@autozsys_mxhvc9
bpool/BOOT/ubuntu_e8m8h0@autozsys_9athz8
bpool/BOOT/ubuntu_e8m8h0@autozsys_61umv1
bpool/BOOT/ubuntu_e8m8h0@autozsys_1q65cz
root@ubuntu:~# 

Then remove them

zfs list -H -o name -t snapshot bpool/BOOT/ubuntu_e8m8h0 | xargs -n 1 zfs destroy

Now, it is ok to upgrade

root@ubuntu:~# zfs list -o space bpool
NAME   AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD
bpool   589M   243M        0B     96K             0B       243M
root@ubuntu:~#