On Feb 7, 2011, at 4:17 PM, NeilBrown wrote:
> On Mon, 7 Feb 2011 15:53:46 -0800 Jeff Klingner <klingner@xxxxxxxxxxxx> wrote:
>
>> I'm planning a backup system for my home server and have run into a question I can't find answered in the mailing list archives or the wiki. Here's the plan:
>>
>> 1. Install system and valuable data on a 3-disk raid1 array (call the disks A, B, and C).
>> 2. Remove disk C, put it offsite. ("offsite" is moderately time-consuming to get to.)
>> 3a. Periodically, remove disk B, take it offsite, and retrieve disk C
>> 3b. Insert disk C, which will be re-synced to gain any changes made since it was removed.
>> 4. Repeat steps 3a and 3b indefinitely, alternating the roles of disks B and C.
>>
>> Thus I hope to get continuous protection against a single drive failure and protection back to the last offsite swap for corrupted or deleted data.
>>
>> I saw in another list message the advice to use two stacked raid1s for this application: http://marc.info/?l=linux-raid&m=126761399008775&w=2
>>> 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
>>
>>
>> Are there important differences between the single 3-disk raid1 array I'm planning to use and this stacked configuration?
>
> Yes. The single 3-disk RAID1 array won't work, the stacked configuration
> will.
>
Goodness, I'm glad I asked! Thank you! I've already done step 1. Do you know if I can transition to the stacked configuration without erasing the disks? That is, generally speaking, can I change the logical arrangement of the devices in raid1 array(s), then re-sync and keep the current (lvm) storage system built on top of the raid intact?
Jeff
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