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Re: Sync two disks and hot swap




On Wed, Nov 08, 2017 at 11:49:33AM +0100, Dominik George wrote:
* A server with two hard drives in removable cases
* A backup process writes data to both disks, making up a live backup server
* A third disk is to be kept off-site
* On a ergular basis, I want to hot-swap one of the disks, as in, remove
  one of the two synced disks and replace it with the stale off-site copy,
  and put the now recent copy off-site

I figure that a simple software RAID 1 would do the trick, but it is not
really made for it and would need some complex manual intervention in
order to not break the state on the removed disk.

Any ideas on how to achieve this, or arguments that RAID 1 would indeed
be a good solution?

I'd personally tend to avoid using an md mirror like this, though it can be made to work. There are some cases in which you might accidentally lose a mirror (e.g., plug the offsite backup in to restore something, have it sync with a newer copy unintentionally) and it requires a bit more effort to do partial restores (like trying to retrieve one deleted file) because of conflicts between the UUIDs on the filesystems as well as having to deal with the mirror itself.

Instead, if you just want a disk that has a readable copy of the files, you may find that rsync is more straightforward and can be a lot faster after the first time if the volume of changes is a small percentage of the total.

Mike Stone