Selon thornber@redhat.com:

quoted

Any insight on where to put the log of the mirror target when mapping over

a

quoted

MD-created raid1 ?

MD effectively uses a log with a single region in it.  So we only need
3 sectors (2 for the header 1 for the bitset) for the log, is there
any space in the MD metadata where this could be placed ?

Nope : the sb is 4096 bytes long, and is full of holes. The biggest one is 128
bytes long, at the tail. We could lower the maximum number of device in a raid
from 27 to 21 to free 3 sectors, but it won't be appreciated and would leave us
with the most fundamental problem : logging in the MD superblock would corrupt
the checksum ... :(