Neil Brown <neilb@xxxxxxx> writes:
> On Tue, 02 Mar 2010 22:50:07 +1300
> Daniel Reurich <daniel@xxxxxxxxxxxxxxxx> wrote:
>
>> Hi Guys.
>>
>> I'm considering implementing a rotating offsite backup solution using
>> raid 1. This solution uses 1 or more internal drives and 2 external
>> e-sata harddrives. The raid setup would be a whole disk partitionable
>> raid1 volume.
>>
>> The idea is that by swapping the external drives, I can have a
>> boot-able ready to run offsite backup of the machine, as well as
>> redundancy on the machine itself. Backups of the important data would
>> be replicated via an incremental daily backup process onto the raid
>> volume itself.
>>
>> The part that concerns me is how to get a clean removal of the drive
>> being swapped out, and how will the raid handle having a stale drive
>> inserted/re-added.
>>
>> I have been considering a couple of ways to handle this:
>>
>> 1) Power the machine down to swap the drives. This has the advantage
>> that the backup is always in a clean bootable state with filesystem
>> consistency pretty much guaranteed.
>>
>> 2) Use mdadm to fail and remove the drives, and then re-add the newly
>> attached stale drive. (Perhaps a udev rule could be made handle the
>> re-add). The disadvantage is this will potentially leave the backup in
>> an inconsistent and possibly un-bootable state unless there is a way to
>> quiesce and sync disk activity before the removal. It will also mark
>> the drive as failed and require It's advantage is that the machine
>> doesn't need to be turned off.
>
> How could it fail to boot?
>
> If your machine crashes, it still boots - right?
> So if you fail the drive at any random time, then it is like a crash, and
> should still boot.
>
> I would:
> sync ; sync; mdadm /dev/mdX -f /dev/whatever
> unplug the device
> mdadm /dev/mdX --remove detached
You can improve on that with lvm and suspending the filesystems. That
ensures that nothing starts just between the sysnc and --fail. But if
you want to suspend / too (in case it isn't read-only) you will need an
tmpfs or ramdisk holding everything needed to suspend and fail the disk.
Having / and /usr read-only will basically garanty a bootable system
alraedy so suspending might be overkill.
> Also, if you want two rotating backups I would create two stacked raid1s.
>
> mdadm -C /dev/md0 -l1 -n2 -b internal /dev/main-device /dev/first-backup
> mdadm -C /dev/md1 -l1 -n2 -b internal /dev/md0 /dev/second-backup
> mkfs -j /dev/md1
>
> Then when you add either device back in, it will just resync the bits that
> have changed since that device was last attached. Make sure you add the
> device to the correct array of course.
>
> NeilBrown
MfG
Goswin
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