Re: Linux RAID migration

[Neil Brown <neilb@suse.de>]

On Tuesday August 7, saeed.bishara@gmail.com wrote:
> On 8/7/07, Neil Brown <neilb@suse.de> wrote:
> > On Tuesday August 7, saeed.bishara@gmail.com wrote:
> > > how can I create raid 5 array that preserves the contents of a given
> > > disk (part of the array)?
> > > I can do that in two steps, raid 5 on two disks then reshape it to be
> > > real raid5, but is there a direct way to do it?
> >
> > What do you mean by "real raid 5".  Two disks are just as much an
> > array as 10.
> I meant by real raid 5 to raid 5 over 3 drives or above.
> > What do you mean by "direct".  If I understand your requirements
> in one mdadm command
> > correctly, there are two obvious steps.
> >   1/ make the data appear in a raid5
> >   2/ change the number of devices in the raid5.
> >
> > Are two steps too many?
> in this case I two options:
> 1. keep the filesystem unmounted tell step 1 in done -> the fs will
> not be available for long time

No, it shouldn't take a long time....

What exactly do you have, and what exactly do you want to achieve?

If think you started of by saying you wanted to convert a raid1 to a
raid5.  Yes, that requires the filesystem to be unmounted, but only
takes seconds to achieve:

 mdadm -S /dev/md0
 mdadm -C /dev/md0 -l5 -n2 /dev/sda1 /dev/sdb1
 mount /dev/md0

If you have a single-disk, you first might need to shrink the
filesystem to make sure there is 128K free at the end of the device:

  umount /filesystem
  resize2fs /dev/sda1 whatever-number
  mdadm -C /dev/md0 -l5 -n1 /dev/sda1 missing
  mount /dev/md0 /filesystem
  # now add the drives and grow
  mdadm /dev/md0 --add /dev/sdb1 /dev/sdc1
  # wait for recovery to finish
  mdadm --grow /dev/md0 --raid-disks=3

something like that..

NeilBrown