zfs q regarding backup strategy

[tech-lists]

Hi,

This is a bit chicken-and-egg for me, in that I might be formulating the
wrong question, and so getting wrong answers, so I apologise in advance 
if I'm being dense.

Basically the end result I want is to be in a position at a backup site
with full backups that can be restored to the live site if needed, 
or even to get just one required file from that backup, given the filename 
and modification time. The backup site has very much more diskspace than
the live site.

I know how to make the full backup and to send it. What I'm looking for
is incremental (or would it be differential) zfs backups (snapshots). 
So let's say an incremental/differential backup to the remote site 
happens every 6hrs.

If 10 incremential backups happen after the full one, and on the
live service someone loses a few files, later adding another few, maybe
a couple of those have the same name as before just with a different
modification time, all the while this is happening 24hrs pass.

How are the accumulated differential/incremental snaps incorporated into
the main backup? Would I need to run something else on the backup
server to integrate the snaps with the full backup? Or would all 
the snaps need to be kept? 

How would one tell if the required files in this example are in one snap
and not in the others?

If the policy was to keep everything in backup regardless of whether it
was deleted or not from the live service (so the backup is like a 
write-only representation) I guess the approach would be different 
to say if the backup just had to be the latest representation of the 
data on the live server. But if it's "keep everything" then what happens
if the live server makes foo.txt then deletes it then makes foo.txt 10
days later in the same place? All the while these
(incremental/differential) snaps have been arriving every 6 hrs.

Additionally the live site is the old zfs on releng/12.2-p10 and 
the backup site is with the OpenZFS on stable/13 which might be an issue
if they handle things significantly differently.

thanks advance for any suggestions or any scripts/tools you can
recommend.
-- 
J.
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