I’m adding a second storage pool to my Incus setup, and it’s on an existing ZFS pool.
I created a ZFS dataset for Incus to use first:
❯ sudo zfs create -o encryption=on -o keylocation=file:///root/key -o keyformat=hex -o atime=off -o recordsize=1M pool/incus
❯ zfs list -o name,mountpoint,mounted,encryption
NAME MOUNTPOINT MOUNTED ENCRYPTION
pool /pool yes off
pool/data /pool/data yes aes-256-gcm
pool/incus /pool/incus yes aes-256-gcm
When I tell Incus to use this dataset to create a new pool, it does, but it also unmounts it and changes the mount type to “legacy”:
❯ sudo incus storage create test-pool zfs source=pool/incus
Storage pool test-pool created
❯ zfs list -o name,mountpoint,mounted
NAME MOUNTPOINT MOUNTED
pool /pool yes
pool/data /pool/data yes
pool/incus legacy no
pool/incus/buckets legacy no
pool/incus/containers legacy no
pool/incus/custom legacy no
pool/incus/deleted legacy no
pool/incus/deleted/buckets legacy no
pool/incus/deleted/containers legacy no
pool/incus/deleted/custom legacy no
pool/incus/deleted/images legacy no
pool/incus/deleted/virtual-machines legacy no
pool/incus/images legacy no
pool/incus/virtual-machines legacy no
Is this expected behaviour? And additionally, I’m kinda curious why? After looking at it a bit, I’m suspecting it’s so the files are mounted under /var/lib/incus/storage-pools/ like they are for my existing pool, but I couldn’t find a definitive answer on Google or these forums so I thought I would ask 
It might be helpful to document this somewhere, initially this confused me a bit. I was also a little surprised by the multiple datasets (e.g. “pool/incus/containers”) and wondering if I did something wrong.
EDIT: Also, if anyone is reading this, don’t do the exact thing I did above — the legacy mount won’t automount on next boot due to the encryption. What I’m doing now (which may or may not be correct) is creating a new dataset for Incus below the dataset I created:
❯ sudo zfs list
NAME USED AVAIL REFER MOUNTPOINT
pool 1.59M 38.4G 42.0K /pool
pool/data 217K 38.4G 217K /pool/data
pool/incus 118K 38.4G 118K /pool/incus
❯ sudo incus storage create test-pool zfs source=pool/incus/test-pool
Storage pool test-pool created
❯ zfs list -o name,mountpoint,mounted,encryption
NAME MOUNTPOINT MOUNTED ENCRYPTION
pool /pool yes off
pool/data /pool/data yes aes-256-gcm
pool/incus /pool/incus yes aes-256-gcm
pool/incus/test-pool legacy no aes-256-gcm
pool/incus/test-pool/buckets legacy no aes-256-gcm
... etc ...