I have a corrupt Reiser filesystem that needs a tree rebuild on it, which can be a scary thing to do (and is only advised when you *really* do need to do it which, unfortunately, I do).
Now, this filesystem largely works, there is just a small part of it that causes problems when accessed. A rebuild could make things a lot worse, or it might just solve my problem (note: my problem appears NOT to be due to hardware failure. rebuilding the tree of a Reiser filesystem on hardware that has badsectors or whatever is VERY likely to make things worse. don’t do it).
So, I’m currently using the filesystem and avoiding the broken bit. I need to know if one: how long a rebuild is going to take, so I can plan the downtime and two: will it complete sucessfully or will the world fall on my head.
LVM snapshots can help here and my filesystem in on a LVM logical volume. The idea is to take a snapshot of the filesystem and run the rebuild on the snapshot. Then you can decide whether you want to take the live filesystem down to rebuild that, or maybe you decide to update your backups best you can and start a new filesystem from scratch.