So you want to migrate your existing Linux partitions to software raid1… I’ve read recently about folks migrating to software raid by actually copying data. I’ve been doing this on-the-fly (sort of), without copying the data, but instead just initializing the partition as an md device and mounting it as such with the data intact. Keep in mind that it needs a slice at the end of the filesystem for the md superblock (with the default version .9), which is why the resize2fs is used. Now, if you want to rearrange how your data is mounted then you’re out of luck, but if you just want to migrate existing partitions to raid partitions, here’s an example with an ext3 filesystem.
started with data on mounted /dev/sdm1, want to add /dev/sdn1 in raid1
##see how many blocks we currently have
tune2fs -l /dev/sdm1 | grep “Block count”
##subtract 64 blocks from the current block count, making space for the md superblock.
resize2fs /dev/sdm1 <blocks>
mdadm –create /dev/md0 –raid-devices=2 –level=raid1 /dev/sdm1 missing
##see that initial data is still there…
mount /dev/md0 /mnt
ls -la /mnt
##add mirror device
mdadm –manage /dev/md0 –add /dev/sdn1
mdadm –detail /dev/md0
##use parted to change partition type to ‘fd’, linux raid auto, make sure the kernel can find it on reboot.
parted /dev/sdm set 1 raid on
parted /dev/sdn set 1 raid on