Document missing disk space on nebbiolo1

[jwokaty]

Our new harddrive on nebbiolo1 sometimes doesn't show all available space:

Filesystem      Size  Used Avail Use% Mounted on
tmpfs            13G  2.6M   13G   1% /run
/dev/sdc2       218G   43G  164G  21% /
tmpfs            63G   16K   63G   1% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
/dev/sdc1       511M  6.1M  505M   2% /boot/efi
data            128G  128K  128G   1% /data
data/home       946G  818G  128G  87% /home                                # This should be 1.7T
tmpfs            13G  4.0K   13G   1% /run/user/1003
tmpfs            13G  4.0K   13G   1% /run/user/1001
tmpfs            13G  4.0K   13G   1% /run/user/1005
tmpfs            13G  4.0K   13G   1% /run/user/1004

This is because there's likely a snapshot taking up space:

jwokaty@nebbiolo1:~$ zfs list -t snap
NAME                   USED  AVAIL     REFER  MOUNTPOINT
data/home@01-03-2023   782G      -      815G  -
jwokaty@nebbiolo1:~$ sudo zfs destroy data/home@01-03-2023
sudo zfs destroy data/home@01-03-2023
jwokaty@nebbiolo1:~$ zfs list -t snap
no datasets available
jwokaty@nebbiolo1:~$ df -h
Filesystem      Size  Used Avail Use% Mounted on
tmpfs            13G  2.6M   13G   1% /run
/dev/sdc2       218G   43G  164G  21% /
tmpfs            63G   16K   63G   1% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
/dev/sdc1       511M  6.1M  505M   2% /boot/efi
data            822G  128K  822G   1% /data
data/home       1.7T  818G  822G  50% /home
tmpfs            13G  4.0K   13G   1% /run/user/1003
tmpfs            13G  4.0K   13G   1% /run/user/1001
tmpfs            13G  4.0K   13G   1% /run/user/1005
tmpfs            13G  4.0K   13G   1% /run/user/1004

From Nikos

zfs is both a volume manager (pool) and a filesystem (zfs)
zpool status will list all pools, here it will list a pool named data consisting of 2 mirrored disks
zfs list will list the filesystems
zfs snap data/home@1-03-2023  will create a snapshot which is essentially a copy of the filesystem at the specified  time
zfs list -t snap will list all the snapshots
zfs destroy data/home@1-03-2023 destroys the named snapshot

It would be good to document this, maybe in the ubuntu documentation since the snapshot sometimes comes back. (Maybe from an OS update?)