Re: RAID 1 | Test Booting from /dev/sdb

[Wols Lists <antlists@youngman.org.uk>]

On 05/05/20 09:11, Stefanie Leisestreichler wrote:
> Hi.
> I want to test if grub is installed on both of the HDs which are part of
> my raid1 array. I wonder which would be the best solution to do so.
> 
> I think I will archive that when I shutdown the computer, make /dev/sda
> powerless and see if it is able to boot from /dev/sdb.
> 
> If it is not booting /dev/sdb will have no changes, I would shutdown,
> connect /dev/sda with power again, turn it on and do a "grub-install
> /dev/sdb". Depending on the state of the array (I guess it will need
> recovery) I would do a "mdadm /dev/md0 --add /dev/sdb1". After recovery
> I would try it again.
> 
> If it is booting from /dev/sdb this HD will have "more" data because of
> the one boot process than /dev/sda.

Why? If it boots to the grub menu, just hit an arrow key to stop the
timeout, and then hold the power button for 5 secs to shut it down again.

> I am not sure if it is a good idea
> to shutdown and just connect /dev/sda with power again, boot (assuming
> /dev/sda is the standard boot medium) because I do not know in which
> state the array will be. What to do in case I do not want to loose data
> from the last boot process with /dev/sdb?

Which would matter why? If you're worried about downloading mail etc,
just kill the network temporarily.

> Change boot medium to /dev/sdb
> and do a "mdadm /dev/md0 --add /dev/sda1" to get it recovered again
> without loosing the "added" data (i.e. in /var/log) from booting? Also
> device identifiers could change I guess. Even if I am fine with loosing
> the "added" data from booting with /dev/sdb, will - when booting again
> from /dev/sda - /dev/sda be the master in the array again?
> 
> It is not clear to me if I understood correctly in which case which
> array member will be the master which will be the base for recovery. Is
> it always the HD one booted from?
> 
The "master" if recovery is required will be the "older" one - in this
case sda because it was disconnected. HOWEVER. Just check whether you
have a bitmap or journal enabled. You can't have both at once, but the
result should be that sda rejoins the array cleanly, raid has a record
of which writes occurred while it was offline, and it will be updated.

> Could you please help me with that?
> 
> Thanks,
> Steffi

Cheers,
Wol