Is there a way to tell if two identically-named ZFS snapshots are actually identical?

Question

How can you tell if two ZFS snapshots are identical or simply named the same?

Say I have two computers named host-a and host-b. Also assume that host-a has a ZFS dataset named /tank/mydataset.

On host-a, I could: zfs snapshot tank/mydataset@mysnapshot' and then I could 'zfs send tank/mydataset@mysnapshot | ssh host-b zfs receive tank/mydataset

This would mean @mysnapshot would be identical on both host-a and host-b.

On the other hand, on host-a I could: zfs snapshot tank/mydataset@mysnapshot followed by ssh host-b zfs snapshot tank/mydataset@mysnapshot.

In this second case, the two snapshots are identically named, but the dataset may be wildly different.

Looking at the properties of tank/mydataset@mysnapshot doesn't list any apparent way of identifying if the snapshots are identical--i.e. a GUID or hash.

Other than trying to take another snapshot of host-a tank/mydataset@mysnapshot2 and trying to send to host-b via zfs send -i @mysnapshot tank/mydataset@mysnapshot2 | ssh host-b zfs receive tank/mydataset is there a way of determining that the snapshots are identical not only in name, but blocks?

Answer 1

There's a property specifically for this use-case.

zfs get guid pool/fs@snap will give you a GUID (Globally Unique ID) for a snapshot. If the GUID doesn't match between two snapshots, one isn't replicated from the other, and they won't work as a basis for further incremental replication.

This is also how syncoid identifies common snapshots to determine replication commands.

(Disclaimer: I am the initial developer and chief maintainer for the Sanoid project, which the Syncoid replication tool belongs to.)