Enhance MIRROR code. Add a few more sanity checks and implement
a zone-based disk selector to make use of both disks when reading.
Also implement a read fail-over. If a read error occurs on one
disk, the I/O is retried on the other.
NOTE: CCD's mirroring support cannot deal with write errors properly
in regards to recovery, meaning that 'old' data under a write error may
be read non-deterministically if you reboot after a write error, and CCD
certainly cannot deal with a disk changeout. And it still can't. Use
vinum if you are really serious about mirroring. CCD basically just
implements a poor-man's mirror.
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