Hi,

Kurt Tunkko wrote on 2008-07-15 00:25:27 +0200 [[BackupPC-users] Howto backup 
BackupPC running on a RAID1 with mdadm for offline-storage]:
> [...]
> *** How to use Software RAID1 to get a 3rd drive for offline storage ***
> This has been tested with Ubuntu Server (in VMWare)
> 
> [...]
> 2. Connect my 3rd harddrive (dev/sdc1) to my machine and add it to the
> RAID1 as spare-drive:
> 
>       # sudo mdadm --add /dev/md0 /dev/sdc1

> 3. Mirror partition tables from one of the existing disks:
> 
>       # sudo sfdisk ???d /dev/sda | sfdisk /dev/sdc

apart from something having been mangled ("???"), I tend to wonder why you
need root permission to read the partition table of /dev/sda but not to write
it to /dev/sdc ;-). You might, of course, have relaxed the permissions on
/dev/sdc, but I'd spare an extra 'sudo' for the howto ;-).

Oh, and I wonder even more why you are first adding the *partition* to the
RAID array and then modifying the *partition table* defining it. If I were
the RAID subsystem, I'd revoke your root privileges for doing that ...

> 4. Disable one of the the two harddrives and remove it from the RAID:
> 
>       # sudo mdadm --fail /dev/md0 /dev/sdb1
>       # sudo mdadm --remove /dev/md0 /dev/sdb1

I believe the original idea is *not* to temporarily cripple your RAID but
rather to add a third disk (three way mirror). I'm not sure if you can do that
after initial creation of the array, but the man page suggests it should be
possible "on kernels which provide necessary support". So you would have
something like

    4. Add new HDD to raid:

        # sudo mdadm --grow --raid-devices=3 /dev/md0
        # sudo mdadm --add /dev/md0 /dev/sdc1

and --fail and --remove only /dev/sdc1, *never* /dev/sd[ab]1.

I'm afraid I don't have any spare HDDs around to test this right now ...

> [...]
> 3) Will it harm my harddrives if I use hot-swapable harddrive-cases?

As long as they don't get *too* hot ... why should it? Whether or not
hot-swapping actually works seems to depend on the driver for the hardware
controller (at least for SATA).

Regards,
Holger

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