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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