How to add new disk to ZFS pool?

[UdoB]

Old thread but can anders's attach be done with a drive of a smaller size?

I have a ZFS mirror pool of 2, 1TB HDD drives and I want to gradually move boot onto 256GB SSD's. I understand this will probably come with a boot storage downgrade to 256G.

As so often: it depends.

• no, you can not add a smaller device as a mirror to an already present disk
• yes, you can add new disks of any size to a pool - creating a new vdev - what is often NOT what you want

You have a pool consisting of a single mirror with two 1 TB disk. Now you want to replace this mirror with two SSDs.

Yes, you can actually do it this way: add the two SSD as an additional mirror to the pool - temporarily. Now you have a "striped mirror", similar to a "Raid10".

Now you can remove the old HDDs from the pool. (Do not just unplug them!) Removing might take some time, be patient. The remainder consist only of the new SSDs. Details are here: man zpool-remove


Be careful, there might be dragons! While I am for sure firm with ZFS I would really test the procedure in a testbed before I do it in production.