Comment 24 for bug 320638

@Phillip Susi / comment #23: Did you actually read what I wrote? :)

I was *NOT* advocating "backup" by having multiple RAID disks constantly connected to the array and in sync. It is completely obvious to me that a hot running copy of data is NOT a backup.
I was advocating the following procedure:
1. Connect disk
2. Wait until it is synced into the array.
3. Shutdown the machine
4. *DISCONNECT* the disc from the machine and consider the completely offline disk as a backup.

This is a backup because the disk is physically disconnected from the machine.
It is much better than a rsync/cp, because it provides a *coherent* copy since all modifications to the data which happen during the copying process are also applied on the backup. With rsync/cp, files which are modified *after* they have already been copied are not up to date in the backup, and for applications which store data in multiple files (which *many* programs do), their data would be corrupt in such a case.

It is relevant to this bugtracker entry because it shows that using multiple kinds of disks in a RAID1, such as SATA+USB, is a common desire and not some exotic border case.
Or can you name any other kind of non-exotic, non-beta (such as btrfs) backup mechanism which can copy data while the system is in use without breaking its coherency? :)