Migrating to gmirrored RAID1

[Ivan Voras]

Henry Karpatskij wrote:
> Hi,
> 
> I have a failing IDE disk which is running my 7.0-p1 server. I've been
> investigating the possible solutions and I've decided to go with two new
> IDE disks and gmirror. However, I'm not too familiar with disk
> internals, I know how to install the system and somehow understand the
> concept of slices and partitions, but that's about it.
> 
> I found some examples on how to install the gmirror on a running system,
> but they all have in common that they just add new spare disk to the
> system and turn on the mirroring to it, but I need to replace the
> current disk which is not the same size as the new ones.
> 
> Any suggestions how one would do such an operation? Should I just
> re-install the server to a new disk, turn on the mirroring and then
> restore the configuration and files from the failing disk? Or is it
> easier to add the disks to the running system, turn on mirroring and
> then somehow dump the current disk to the mirror and then re-configure
> it to boot from the gmirror and remove the failing disk?

If the new drives have the same size as the old one, you could do it
gradually:

- Mirror the old drive to new drive #1
- Replace the old drive with new drive #2, mirror it from new drive #1

If the sizes are different, this won't work and the best would be to
install a fresh system on a new drive and transfer the configuration
from the old one.

Booting will always happen from a single drive. Since up to the point
file systems are mounted all relevant drive access is read-only, the
BIOS will happily boot one of the mirrored pair, then the kernel will
pick up and activate mirroring before file systems are mounted.

This is why all instructions start with one drive and later add a new
drive to hold the mirror; it's easy to set up a single drive and then
simply mirror it.

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