Re: migrate single disk to RAID 1?

[Michal Soltys <soltys@ziu.info>]

Ben Beuchler wrote:
>>> umount /dev/sdb4
>>> mdadm --build /dev/md0 --level=1 -n2 /dev/sdb4 missing
>>> mount /dev/md0 /mail
>>> mdadm --add /dev/md0 /dev/sda4
>>>
>>> Will that work?  Or am I misunderstanding something fundamental?
>>
>> It'll work but you'll be using a legacy array without superblocks, so you
>> can't have various features that come with superblocks
> 
> Right; for this particular application I'm willing to give up
> superblocks.  Just to be 100% sure, thought:  the way I've described
> the process would be non-destructive of the data on sdb4?
> 
> Thanks!

Actually, if on that sdb4 is a shrinkable filesystem, you can create md 
raid with superblock at the end. So the process would be something like:

- resize2fs filesystem on sdb4 a bit (shrink)
- mdadm stuff with e.g. 1.0 superblock
- resize2fs filesystem again, this time on md0

According to md(4) you shouldn't need to shrink more than 128KB, but be 
sure to get a healthy margin. Or do simple test with some tiny 
partition, or md-on-lvm.