@VersionMonkey - Thanks for the feedback, and had to laugh when I realised that asking this question has resurrected a 5 year old thread. Whoops!
So are we saying then, that running two 10TB NAS drives in RAID 1 for instance, won't make your data recoverable if one of those disks were to fail? For argument's sake, simple folder / file systems holding media, music, photos. If that data is unrecoverable, what's the point of mirroring?
I understand your point on ransomware, although a lot of people have backup drives permanently connected to their PCs with scheduled backups, and I'd argue they'd be susceptible to the same problem. I disconnect my backup disk afterwards and store it away for this reason.
I also understand that you can't count a mirror as THE backup, which is what the 3,2,1 backup rule is about, but also can't be considered ONE of those backups?
Also understand about the incremental and version controlled backup model, but that's just one interpretation of a backup isn't it? Copying files over to a USB stick and putting them in a drawer is considered a backup, even if not as advanced as a professional backup system like Time Machine, et al.
Cheers for the help so far.
Ah yes, I also didn't spot the thread resurrection. Whoops indeed.
But anyway. Think of it this way.
You should view a RAID as if it is a more reliable/higher performing single disk. All the things that can go wrong with a single disk other than outright failure can also go wrong with an array
and actually outright failure isn't completely eliminated either.
You format the drive and you write some files to it, but while the files are being written the power goes out and the filesystem is left in an inconsistant state. You run chkdsk, but that
says it can't fix it - your data is unrecoverable (at least without resorting to some other data recovery tool); but a mirror won't help at all because all you have is two copies of a broken
filesystem. A Windows Update causes photoviewer to overwrite the first half of every image file you open; a mirror won't help you get your photos back because all you have is two copies
of the corrupted files.
While outright failure may (in some configurations) be less likely with a RAID (that power failure could also mess with the RAID's metadata don't forget), it doesn't address all the other things
that can go wrong with storage.
RAID is there to allow you to carry on working while you order a replacement disk (while hoping that you don't get a second disk failure before the array is rebuilt - if you bought all the disks at
the same time, that's actually more likely than you might wish for).
To quote backblaze: "A 3-2-1 strategy means having at least three total copies of your data, two of which are local but on different mediums (read: devices), and at least one copy off-site". It's
the "read: devices" bit that tells you you shouldn't count a mirror as providing the two local copies: things that can go wrong with one side of the mirror will be replicated immediately to the other
side of the mirror with the sole exception of a hard failure of one of the disks. You should still have a completely independent local copy and then a 3rd (or more) off-site copy. So, yes you can count
mirror as one copy; but not because it is a mirror - you can count a single disk as one copy too. The key with that scheme is that the two local copies are independent - a mirror or other RAID doesn't
create independent copies because it is still a single filesystem.
The full, incremental and differential concepts are about balancing the storage requirements for the backup and how long they take vs how much time it would take to do a restore.
How you approach backup depends on how important the data/system is to you and how quickly you need it back when you accidentally delete your final thesis the day before it is due to
be submitted. If that's what you are trying to protect against then a copy on a USB stick in a drawer is possibly a viable backup strategy but RAID isn't.
