Observations about geom mirror and slices

[Danny J. Zerkel]

Just a note about geom mirror and slices.  I don't know how many
people have run into this yet, but a warning may be in order in the
man pages.  When I first started experimenting with mirroring on
my system, I naively thought I could just mirror any disk partition.
Turns out this can be very bad.  The problem is that if you mirror a
partition that extends all the way to the end of the slice, then geom
mirror will think that the mirror part is the entire slice.  This is
because geom mirror uses the last block for status information.  When
geom starts tasting the disks, geom mirror sees the last block of the
whole slice before it sees the individual partitions.  To avoid this,
none of the partitions, except c, should include the last block of the
slice.

Danny J. Zerkel
dzerkel at columbus.rr.com