I created a subdirectory and put a mirrored file in it. When I tried to migrate the mirrored file to another MDT, it returned an error migrating the mirrored file that I didn't expect:

tests# lfs mkdir -i 0 /mnt/testfs/dir1
tests# cp /etc/hosts /mnt/testfs/dir1
tests# lfs mirror extend -N1 /mnt/testfs/dir1/hosts
tests# lfs migrate -m 1 /mnt/testfs/dir1
lfs migrate: /mnt/testfs/dir1/hosts migrate failed: Operation not supported (95)
tests# lfs getstripe -m /mnt/testfs/dir1
1
tests# lfs getstripe -m /mnt/testfs/dir1/hosts
0

So it seems like the directory was migrated, but the mirrored file was not. Is there some reason that the mirrored file could not be migrated? I don't think that should be any different than migrating some other composite file.