according to http://linas.org/linux/raid.html:
(as of kernel 2.6.26 in July 2008):


   - For a RAID-1 config with two disks, if the data mis-compare was due to
   one block being unreadable, then the copy will be made from the remaining,
   readable block, which should hold good data. If both blocks were readable (
   *i.e.* were read from the disk, without the disk indicating any sort of
   error condition), but there was a mis-compare, then the data from the
   highest-numbered disk is copied to the other disk. This results in a 50-50
   chance that good data was over-written by bad. Furthermore, this is done
   silently: no syslog messages indicate either a mis-compare, or that a
   repair action was taken!
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