Hi,

Kurt Tunkko wrote on 2008-07-16 01:05:30 +0200 [Re: [BackupPC-users] Howto 
backup BackupPC running on a RAID1 with mdadm for offline-storage]:
> Holger Parplies wrote:
> 
> >> 3. Mirror partition tables from one of the existing disks:
> >>
> >>    # sudo sfdisk -d /dev/sda | sfdisk /dev/sdc
> [...]
> 
> I took the information above from the page:
> 
> Setting up software RAID in Ubuntu Server
> http://advosys.ca/viewpoints/2007/04/setting-up-software-raid-in-ubuntu-server/
> 
> " [...]
> For example, to copy the partition table from the second 
> drive ???sdb??? onto the first drive ???sda???, the sfdisk command is as 
> follows:
> 
> sfdisk ???d /dev/sdb | sfdisk /dev/sda"

my point is that your pipeline consists of the two commands

        sudo sfdisk -d /dev/sda
        sfdisk /dev/sdc

connected by a pipe. It's not as though 'sudo' somehow magically applies to
the pipe and not only its own parameters. You need either

        sudo sh -c "sfdisk -d /dev/sda | sfdisk /dev/sdc"

or rather

        sudo sfdisk -d /dev/sda | sudo sfdisk /dev/sdc

Similarly, you can

        echo 'backuppc hold' | sudo dpkg --set-selections

because you don't need root privileges to echo 'backuppc hold', and my
favorite for I/O-redirection with root privileges is

        some-command-that-generates-output | sudo dd of=/root/output.file

Sorry, that was off-topic.

> I don't know if it's possible to add a 3rd drive to the RAID, that 
> hasn't got the right partitions on it :-?

You're not adding a drive to the RAID, you're adding a partition, i.e. a block
device consisting of at least N blocks, where N is the size of the smallest
partition currently in the array. You usually choose identically sized
partitions to avoid wasting space. If you've got identical disks with only a
single partition, you can probably use the disk (/dev/sd[abc]) instead of a
partition (/dev/sd[abc]1) and put the first track to good use. It might be
desirable to have the partitions of one RAID array in the same place on all
disks (identical R/W speed, presuming identical disks) or it might not. You've
got a lot of freedom.
Of course the partition needs to exist and be large enough though.

> [...]
> While this approach is much better than the one I suggested yesterday it 
> still leads to some Questions:
> 
> 1) Is there a way to add the 3rd drive to RAID1 as soon as it will be 
> connected to the system (External harddrive that is oonnected via usb2)?
> Or more generally: Can I run a script when an external storage device is 
> connected via usb?

Yes. Are you sure you want that Windoze-type-oh-no-feeling when you plug a USB
stick with valuable data into the computer and it automatically fills it with
the first few GBs of your BackupPC pool? :) Not sure if sfdisk will allow the
partition table to exceed the device size though ...

man udev && good luck

> 2) Do I need to resize my RAID after removing the 3rd harddrive with
>     # sudo mdadm --grow --raid-devices=2 /dev/md0
>     Are there any problems when the RAID will be used in clean, but 
> degraded state?

I haven't had any so far with one-way-mirrors. You might get an occasional
warning e-mail. Otherwise, I wouldn't expect any difference between
non-degraded two-way and degraded three-way mirrors with two operational
disks.

Finally, you might take a look at the mdadm --write-mostly option. Sounds
as if it were meant for cases like this one.

Regards,
Holger

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