What situations is using a snapshot preferable to rolling back an update or software install? What data does a snapshot store? Is it configuration files? How does it differ from raid?

Thanks for your expertise!

For example if, for reasons unknown, you can't provide a clone to work on that instead? If you're in a troubleshooting cycle of testing invasive changes you have no idea of if a result will stick? Snapshots basically allow you to "freeze" the current state of a system in turn allowing you to work (relatively) safely on a Live system. Care should be taken though especially with Production systems that store data that should be siphoned off before restoring to the previous state.


Depends on the method used (VMware, ZFS, etc, etc): everything (disk and RAM) up to the point of the snapshot, difference between snapshot and previous disk image, etc, etc.


What kind of "snapshot-like" functionality does RAID offer?

RAID just makes a duplicate of your personal storage right so RAID would not backup system files then.

RAID != backup. Ever.

This is not how a RAID works.

http://en.wikipedia.org/wiki/RAID
http://en.wikipedia.org/wiki/Standard_RAID_levels
http://searchstorage.techtarget.com/definition/RAID

Apologies if it's unsolicited but RAID 5, for example, is perticularly not a backup and not worth the discs it's written upon.

That's just confused. RAID does do something about disk failures, if done right. But that isn't a backup. Imagine, for example, you have an important file. If you delete the file, in the normal way, it will deleted on the primary storage, whether that storage uses RAID or not. You can get it back from your backup, but not from your RAID. And, in a fire or a flood, it is only your off-site backup that has a good chance, and your RAID probably doesn't have off-site disks.

So, what RAID protects you against (and then not all modes and configurations) is different from what Backup protects you against.

Whether RAID does anything about your system files depends on whether you have configured the system to keep those files on the RAID. You could choose either way, so there is no simple 'RAID does this' assertion that emerges, because it could be either, depending on configuration.

Note also that RAID 0 makes no attempt whatsoever to make your data more secure, and, in fact does the opposite, so that's another case in which 'RAID does this...' statements are a bit dangerous.