ZfS & GEOM with many odd drive sizes

[Bakul Shah]

>                 If you do that, ZFS can use its checksums to continually
> monitor the two sides of your mirrors for consistency and will be able
> to notice as early as possible when one of the drives goes flakey.

Does it really do this?  As I understood it, only one of the
disks in a mirror will be read for a given block.  If the
checksum fails, the same block from the other disk is read
and checksummed.  If all the disks in a mirror are read for
every block, ZFS read performance would get somewhat worse
instead of linear scaling up with more disks in a mirror.  In
order to monitor data on both disks one would need to
periodically run "zpool scrub", no?  But that is not
*continuous* monitoring of the two sides.