Re: raid1 with rotating offsite disks for backup

[Jeff Klingner <klingner@stanford.edu>]

>>>> 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



On Feb 7, 2011, at 5:19 PM, NeilBrown wrote:
> I'm surprised that you found the correct advise, and chose not to follow
> it .....

Thanks again for your advice.  I didn't search the list until after completing my initial setup; my mistake and my lesson learned: research first.

> The array you will end up installing on is (slightly) smaller than than the
> array you have created.  So no: you cannot re-arrange things - you have to
> start again.
> 
> 
> (well, to be honest, it is possible that you could resize whatever is on the
> array and then build the secondary array and make it work.  But that path is
> error-prone and this margin is not large enough to properly describe the
> process).

Reckless fool that I am, I'm going to try it anyway.  Here's my rough plan:

1. Degrade the 3-disk raid1 array by removing two disks, leaving only one, and leaving the array running.
2. Set up the first 2-disk array (md0 in your description) de novo on the two freed disks
3. Move my data from the original raid1 array (with only 1 of three disks present) to the new 2-disk md0 via lvm mirroring
4. Free the third disk by finally taking down the original raid1 array
5. create the second 2-device array (md1) using md0 and the freed third disk.  Somehow ensure that the md0 component has primacy for syncing.

Jeff