Linux LVM – How to Commit or Revert a Snapshot

From the lvconvert manpage:

lvconvert will change a linear logical volume to a mirror logical volume or to a snapshot of linear volume and vice versa.

Emphasis mine.

So yes, it should be possible to convert a snapshot into a linear LV or mirror. If that means you can mirror a snapshot and then use it as a linear lv, that's something you would have to try out.

Apparently, the manpage and me we wrond :P I remembered seeing this stuff in the manpage, but I hadn't actually tried converting a snapshot to a linear LV. Having seen the comment below, I decided to check it out. From what I can see now, it is not possible, what ever the manpage may imply, to convert a snapshot to a linear LV. What is possible using lvconvert is to convert a mirror volume to a linear LV. I think the manpage should be edited a bit here.

If someone does find a way to do this, let me know, but from what I know now, I'd say: not possible. Kinda logical when you think of it, because converting a snapshot LV to a linear LV means something in the line of

dd if=linear of=snapshot

Otoh, you can use a snapshot as a logical volume by itself. As I explained here, LVM is just some device mapper magic. So if you would take an LVM snapshot and then use that for your experiments, the original disk would not be touched, but can still keep functioning normally at the same time.

Why not have a look at the snapshots section of the LVM-HOWTO?

LVM snapshots are your basic "copy on write" snapshot solution. The snapshot is really nothing more than asking the LVM to give you a "pointer" to the current state of the filesystem and to write changes made after the snapshot to a designated area.

LVM snapshots "live" inside the volume group hosting the volume subject to the snapshot-- not another volume. Your statement "...lots and lots of unallocated freespace not it the partition" makes it sound like your thinking is that the snapshots "live" outside the volume group subject to snapshot, and that's not accurate. Your volume group lives in a hard disk partition, and the volume being subject to snapshot and any shapshots you've taken live in that volume group.

The normal way that LVM snapshots are used is not for long-term storage, but rather to get a consistent "picture" of the filesystem such that a backup can be taken. Once the backup is done, the snapshot is discarded.

When you create an LVM snapshot you designate an amount of space to hold any changes made while the snapshot is active. If more changes are made than you've designated space for the snapshot becomes unusable and must be discarded. You don't want to leave snapshots laying around because (a) they'll fill up and become unusable, and (b) the system's performance is impacted while a snapshot is active-- things get slower.

Edit:

What Microsoft Volume Shadow Copy Services and LVM snapshots do aren't too tremendously different. Microsoft's solution is a bit more comprehensive (as is typically the case with Microsoft-- for better or for worse their tools and products often seek to solve pretty large problems versus focusing on one thing).

VSS is a more comprehensive solution that unifies support for hardware devices that support snapshots and software-based snapshots into a single API. Further, VSS has APIs to allow applications to be made quiescent through the snapshot APIs, whereas LVM snapshots are just concerned with snapshots-- any quiescing applications is your problem (putting databases into "backup" states, etc).