zfs backup question

[Julien Cigar]

Hello,

We are backuping our servers with Sanoid / Syncoid.
Our servers provde access to several (web) applications and each
of them is stored in a dedicated ZFS dataset and running in a
dedicated jail.
So we have something like zroot/jails/app1, zroot/jails/app2,
zroot/jail/appx, etc

Due to some changes to our CI/CD pipeline and the way our apps are
deployed, the zfs dataset (zroot/jails/xxx) is now destroyed and
re-created on each deployment (the "app" is distributed as a gzipped
zfs dataset and the deploy process is a simple send/recv).
It works well except for backuping as there is no common ancestor
anymore each time the dataset is destroyed.
The answer is probably "no", but: is there a way to handle this
without having to send the whole dataset when backuping?

(We are thinking of maybe use Podman (or maybe simply a FreeBSD
package?) to package/distribute the application but I haven't found
any good documentation on how packaging something with Podman yet ..)

Thanks,
Julien

--
Julien Cigar
Belgian Biodiversity Platform (http://www.biodiversity.be)
PGP fingerprint: EEF9 F697 4B68 D275 7B11 6A25 B2BB 3710 A204 23C0
No trees were killed in the creation of this message.
However, many electrons were terribly inconvenienced.

[Karl Vogel]

>> On Fri 19 Jun 2026 at 05:00:31 (-0400), Julien Cigar wrote:

> Is there a way to handle this without having to send the whole dataset
> when backuping?

It might be easiest to compare hashes of the old and new files to create
your backup. I made two directories with some HTML files: b-old (old
backup) and b-new (production, which will become your current backup).

me% ls -lF
drwxr-xr-x 2 vogelke wheel 3264 19-Jun-2026 06:56:12 b-new/
drwxr-xr-x 2 vogelke wheel 3264 19-Jun-2026 07:01:03 b-old/

I copied b-old to b-new and made some minor changes to b-new:

* removed b-new/1.htm
* added b-new/2.htm
* changed b-new/3.htm

I hashed the files, sorted by filename and compared:

me% cd b-new
me% md5sum * | sort -k2,2 > ../b-new.sum
me% cd ../b-old
me% md5sum * | sort -k2,2 > ../b-old.sum
me% cd ..

me% diff b-old.sum b-new.sum
41a42
> 8f9ea97ad8024ff88c1b39a518e51838 2.htm
43c44
< eeaf2f41b27f5fe6cffc6342d016ff84 3.htm
---
> 6c807fc0929464b31b432563bfce5be0 3.htm
45d45
< 0e3876813506bf815a76d60adb1fe8db 1.htm

You can copy your old backup and update these files when creating your
current backup; the deleted files simply won't exist. If you're also
creating directories, that would be more complicated. I don't know if
you can run something like "syncthing" between two existing directories
in their own jails.

--
Karl Vogel I don't speak for anyone but myself

The server room doubled as the marketing dept's wine cellar.
I no longer work there. --Reddit "janky setup" comment, 6 Jun 2026