Questions about snapshot and replication

[Stephen J]

Okay, I am setting up cross replication of snapshots between two freenas boxes and I have three questions I just can't find answers to so I hope someone can help.

Recursion
Okay, I have had my Freenas snapshot both ways and when it is recursive, it created a snapshot of each sub data set beside the primary tank. What advantage or disadvantage is there in recursion? I can see having a snapshot of a storage sub-unit could be helpful if you just want something out of that specific zvol or something. Is there a disadvantage?

Lifetime
So, if you set this to 2 weeks, what happens when two weeks comes for your original snapshot that got replicated that is the basis for all the other ones on your replication target? Do the replicated snapshots not expire? I have this feeling that I still don't understand some key aspect of snapshots and replication.

Stale Snapshots
In the replication setup it has the option to delete stale snapshots. What is a stale snapshot and how does it relate to the above questions? Should this be checked or not?

Thanks,
Stephen

p.s. These questions are really bugging me and holding up actually putting these units fully online.

 

[Joshua Parker Ruehlig]

I believe zfs if smart enough to be able to delete snapshots without effecting other snapshots. At least in my experience as long as a snapshot is saved it makes sure whatever it needs is never really 'deleted' and it grows accordingly. At least in my experience, don't quote me though, test it.

 

[Stephen J]

Thanks Joshua for the input. Unfortunately, I don't know that I gained much clarity on the issue.
Is there no one here that understands snapshots and replication well enough to answer this?

Thanks,
Stephen

 

[Joshua Parker Ruehlig]

I don't understand what a stale snapshot is but I hope I can at least clarify that deleting a snashot does not ruin newer/older snapshots. newer snapshots aren't based on older snapshots, newer snapshots are based on your current dataset and a set of delta's which is what has changed. older snapshots are based on newer snapshots and a set of deltas. When the newer snapshot is deleted both sets of deltas are merged into the older one. Deleting an older snapshot deletes deltas between the older snapshot and the newer snapshot. Lifetime allows FreeNAS to delete snapshots after a period of time to save space (deleting deltas).
Basically don't worry about deleting snapshots, zfs will always let you revert to a snapshot as long as it is not deleted.

Hopefully your 'Lifetime' question is at least answered, LOL.

 

[Stephen J]

Thanks Joshua, I think that answered the lifetime question.

Stephen