I note that trixie-backports currently has version 2.4.2-1~bpo13+1 and that the release notes for 2.4.2 say that it supports Linux kernels up to 7.0.
trixie-backports currently has kernel 7.0.4 so that’s all good. However, just a few days ago I checked and trixie-backports had ZFS 2.4.1 which listed compat up to kernel 6.19 meanwhile trixie-backports at that time did already have kernel 7.0.4. So, at that time technically if getting your kernel from backports as well as your zfs, you would be using an unsupported kernel version,right?
Presumably agiven this is DKMS it probably would have still compiled but may not have operated correctly, since it would be an unsupported configuration.
If choosing to use Debian zfs packages from backports, is it inadvisable to also use the kernel from backports due to periods where the kernel packages are ahead of zfs support? Or is that overly paranoid?
I think that’s a fairly reasonable level of paranoia, honestly. The problem is that stuff in backports is by obvious necessity less well tested with any random extra thing from outside the main repo that you might throw at it, so you can wind up with mismatches there where you would not have had a mismatch between backports and main.
With that said… actual issues from this kind of thing are fairly rare, and in my experience quite minimal. Most commonly, you’ll see something like replication refusing to work until you find and sort out your problem.
So basically, if you really want the backports kernel, I think you can pretty reasonably decide that you’re willing to tolerate an occasional kernel/userland version mismatch issue, if you’re prepared to notice it, recognize it, and if it causes you problems, manually make a change when the problem crops up.
But if you really don’t want to deal with that kind of potential issue, relatively minor and fixavle though it might be, that is also a reasonable proposition, and in that case, no, I would not recommend running both the kernel and the zfs userland out of backports, for exactly the reason you’ve already spotted.
The zfs code prevents compilation for unsupported kernel versions.
No, you can easily stick to the default debian kernel and still use zfs from backports (this is what I do). This works because the stock debian kernel is an old enough LTS kernel.