ZfS & GEOM with many odd drive sizes

[Doug Rabson]

On Wed, 2007-07-25 at 10:47 -0700, Bakul Shah wrote:
> >                 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.

This is of course correct. I should have said "continuously checks the
data which you are actually looking at on a regular basis". The
consistency check is via the block checksum (not comparing the date from
the two sides of the mirror).