Root on ZFS - consider large boot partition (Debian)?

[mbentley]

At least for the Debian guides, I have found that there are situations where the suggested 1G default for the boot pool may not be sufficient. While 1G sounds fine and is large enough for doing the basic install, I've found a scenario where it is less than ideal.

In the Debian guides it suggests doing:

sgdisk     -n3:0:+1G      -t3:BF01 $DISK

So the scenario I found myself in

• I take hourly, daily, and one weekly snapshot of my system zfs partitions.
• I had three different kernels installed on my system (unattended-upgrades installed three - I just hadn't rebooted and cleared any old kernels)
• Then a zfs package upgrade was available.

With three kernels, that meant that I had three initrd files of ~85MB each, there were snapshots of those and then the initrd re-generation process takes a backup file to the /boot partition so it is essentially storing 3 from when the kernel was installed with the old zfs package in a snapshot, 3 backup initrd files, and then 3 more newly generated initrd files which in itself is using over 750MB. Long story short, I ran out of disk space, leaving the system in a tricky state where the initrd wasn't re-created so if I had rebooted, I might have been in a bad situation.

I was able to get myself back into a fine state but it just involved a bit more manual user intervention that I wanted. For now, I just disabled kernel updates via unattended-upgrades since I don't automatically reboot anyway so at least I won't be getting new kernel packages and 1G is still fine for a single kernel + snapshots if I need to re-generated my initrd, like in the case of a new zfs package but I'm somewhat now stuck with my current layout without doing some surgery to change it.

I'm not sure if changing the default of 1G makes sense but would it make sense to add a sizing consideration note to increase it if one intends to automatically snapshot that boot partition? I know this is something that sounds obvious but I regularly forget about the space that snapshots take up when thinking about sizing and being someone who only had a single partition in my previous, non-zfs on root setup, I wasn't as familiar with the disk space challenges that might come up since I had my whole root partition's space available for /boot previously so sizing /boot wasn't much of an issue in the past for me. I'd be happy to create a PR for that on the Debian guides if that seems like a logical approach.

[rlaager]

I think the recommended size of that partition has inched up over time for this reason. There is, of course, a trade-off. But, on the other hand, drives are bigger all the time.

I'm not opposed to a note in the guide. A great version of the note would indicate the approximate size that each kernel takes (which you've included here), so people would have an idea of how to calculate their desired size.

[mbentley]

Thanks for the guidance there. I created #297 with a note. I tried to keep it as brief but informative as possible to help guide sizing.