Creating partitionable raid on existing disk (was: Re: Partitioned raid and major number)

On 2004.03.01 02:09, Neil Brown wrote:

On Monday March 1, miquels@cistron.nl wrote:

quoted

What do you think of that approach ? Converting from a 1 disk setup
to a 2-disk RAID1 setup on an existing system is something that lots
of people want to do, seeing that the software raid howto even has
a few paragraphs dedicated to it.

What I am thinking of doing is allowing:

   md=0,1,/dev/sda

to assemble a raid1 array without a superblock which uses just
/dev/sda.
Then

   md=d0,1,/dev/sda root=/dev/md_d0p1

would boot off md/d0p1 instead of sda1, but it would be the same data.

Then you would be able to add mirrors to this with something like:

  mdadm --grow /dev/md/d0 --disks=2
  mdadm /dev/md/d0 --add /dev/sdb

and you could convert it into an array with a persistent superblock
using:
   mdadm --grow /dev/md/d0 --persistent=yes

The only difficult bit is the setting a persistent superblock means
reducing the size of the device, and I would like it to be hard to do
that in error, but not impossible to do it.

Any progress on this?

The right thing to do would probably be to check if the device actually has
partitions - refuse to reduce the size of the device if it doesn't have
any partitions, or if any partition overlaps with the MD superblock.
That should be easy enough I think.

But it would take changes in both userlevel tools (add --grow to mdadm) and
the kernel, right ? So in the short run, I'll be better off by booting a
Knoppix CD and running mdadm from there, I suppose.

Are you interested in the patch I made to be able to initialize (but not run!)
an array on a disk that is otherwise busy (i.e. can't be bd_claim()ed) ?
Basically it just adds a SET_ARRAY_INFO_CONFONLY ioctl. I have patches for
both the kernel and mkraid, and they're pretty simple.

Mike.