On Tuesday February 19, micah@riseup.net wrote:
> I've got /dev/sda with data on it, I want to add /dev/sdb on as a
> mirror (raid-1), can I do this without destroying data on /dev/sda
> when creating the raid? I've setup /dev/sdb to have all the same
> partitions and sizes as /dev/sda has, and I've setup a raidtab file so
> that all the devices match up, but I am a little paranoid about
> creating the raid because I dont want to destroy data on /dev/sda.
No and yes.
If you have data in /dev/sda1, and you create /dev/md0 as a RAID-1
array of /dev/sda1 and /dev/sda2, then /dev/md0 with by 64K smaller
than /dev/sda1 (due to the RAID superblock) and you will loose any
data that it there.
What you could do is create degraded raid arrays on /dev/sdb:
e.g.
mdctl --create /dev/md0 --level=1 -n 2 /dev/sdb1 missing
and then copy the data access at the filesystem level
mkfs /dev/md0
mount /dev/md0 /somewhere
cp -a /whereever /somewhere
and the unmount /dev/sda1 and add it to the array
umount /dev/sda1
mdctl --add /dev/md0 /dev/sda1
This will work, but requires copying the data.
An alternative is to use my md partitioning patch, from
http://www.cse.unsw.edu.au/~neilb/patches/linux-stable/2.4.17/
The patches you will need are RaidSplit, MdPart and possible MdpMajor.
Depending on what partitioning tool you used, it is quite possible
that the last few hundred K of sda is not part of any partition.
You can check this by using "fdisk /dev/sda", and giving commands "x"
and "p".
Then look at the start and size of the last partition, add them
together and divide by 2 (to convert sectors to K).
Then compare this with the size of /dev/sda as reported by
/proc/partitions.
If the different is more than 64, then you have room for a RAID
superblock that wont overwrite any data.
So:
mdctl -C /dev/md0 -l 1 -n 2 /dev/sda missing
Then stop using the sd partitions and start using the md partitions.
/proc/partitions will tell you the major and minor numbers of
the md partitions.
You can then hot-add /dev/sdb with
mdctl -a /dev/md0 /dev/sdb
and you will have all your data on all your partitions on a raid-1
array.
NOTE: The actual calculation to see if there is enough space for a
raid superblock is:
E <= ((S-1) % 64) - 64
when E is the end of the last partition, and S is the size of the
drive.
i.e. reduce to a multiple of 64 and subtract 64.
NeilBrown
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