Yeah, my oldest vdevs were configured with 512B block size, because that was the default and the ZFS community wasn't being particularly loud about ashift=12 being a good idea until later. As far as I know, there is no easy way to solve that problem? Is it possible to replace one vdev with another? Off the top of my head, my understanding is that you can replace individual disks, but replacing entire vdevs isn't possible?
If it is possible, yeah, I'll definitely replace the older vdevs entirely with new ones that have better ashift. And, while I'm at it, I'll probably switch to all 6-disk raidz2s, since that's another thing that I only learned too late, that raidz2 works best with an even number of disks in the vdev...
AFAIK the block size is a pool wide setting. So you can't even mix and match block sizes, let alone incrementally upgrade it. And what's more, you can't even change the pool wide setting - so the only solution is to create a new pool and rsync your data up (which is just horrible!)
However I've only done minimal investigation into this issue so if you do find a safe way to upgrade the block size then I would love to know (I'm in a similar situation to yourself in that regard)
It's not pool-wide, it's vdev-specific. You can add new vdevs with more optimal ashift values. With my pool, half of the vdevs have sub-optimal ashift, the other half are good.
If it is possible, yeah, I'll definitely replace the older vdevs entirely with new ones that have better ashift. And, while I'm at it, I'll probably switch to all 6-disk raidz2s, since that's another thing that I only learned too late, that raidz2 works best with an even number of disks in the vdev...