[laumars]

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)

[jspiros]

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.

[laumars]

That's handy to know. Thanks