[rescue] expanding a ZFS pool: leave slack space?

[Peter Corlett]

On Fri, Apr 30, 2021 at 12:42:44PM -0500, Scott Newell wrote:
[...]
> My concern is that in the future a replacement drive might not be exactly
> the same number of sectors, and if it's too small, the mirror obviously
> won't accept it.

The number of sectors for a given capacity disk is standardised, and has
been for over a decade:

http://www.idema.org/wp-content/downloads/2169.pdf

The standard exists exactly because disks used to have slightly different
sizes and this was a problem for disk replacement in RAID. It is now no
longer a problem unless you're importing disks from some other universe
rather than getting them from WD and Seagate like the rest of us.

Since a 4TB disk is actually slightly larger than 4TB, you could always
create a partition which is exactly 4TB and use that. ("zpool create"
actually creates a partition table if you hand it a full disk anyway, but
you don't get to decide the exact capacity.)