On Fri, 2024-01-26 at 16:27 +0000, Michael Kjörling wrote: > On 26 Jan 2024 16:39 +0100, from h...@adminart.net (hw): > [...] > > Having multiple generations of backups already increases the needed > > storage space by a bit more than half. That makes it already arguable > > if it's better to make (multiple generations of) backups on a single > > RAID or on N single disks. Any of the disks can fail at any time. If > > you go with N == 2, a RAID (with multiple generations of backups on > > it) can be better because when a disk fails, the RAID will very likely > > survive and the non-RAID may not. > > I'm not sure how you figure that.
It's simple: when using RAID1 with 2 disks, you double the physical storage capacity needed. When using 2 independent disks, you also double the capacity. In either case, when a disk fails, the RAID has a chance to survive the failure while the single disks don't (ignoring that you may be able to recover data from a failed disk). So either you don't loose a backup or you do loose one backup. If you're lucky, you loose the outdated backup rather than the most recent one. If you made the backup on RAID, you don't loose the most recent backup. I like it better not to loose the most recent backup. That's assuming that you have storage capacity for a single backup, i. e. one backup on RAID vs. one most recent backup on one disk and on older backup on the other disk. Of course, when you have multiple generations of backups on each set of disks, things get more complicated. It also gets more complicated when the volume you're making backups of doesn't fit on a single disk ... > [...] > > Trying to make things appear easier by pointing out that failed disks > > can be replaced is not helpful. > > It's a _backup_. _By definition_, a backup is only critical once the > primary copy becomes inaccessible for some reason. Hence: I have to disagree here. The backup is always critical before you have eliminated the possibility that the data can get lost. Only when you have done that, then you don't need a backup at all.
