2

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?

4

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.)

1
  • Ooh...syncoid....you may have solved two of my problems with one answer. I'll have to check it out. Thanks! :) Jan 17 '20 at 21:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.