Updating linux lenny
to the main LV and the snapshot stores a copy of the old block ?
or to the snapshot LV while the main LV remains untouched ?
If it is the lowest snapshot (or the only one), the pool just gets dropped and the operation is very fast.
Some file-systems do offer in-filesystem snapshots, ZFS and BTRFS are but two of the better known ones.
James pointed out one of the faults of this system.
When you have multiple snapshots of the same volume, every time you write to a block in the main volume you potentially trigger writes in every single snapshot.
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.
If the dropped snapshot is part of a snapshot tree, some blocks will be copied to lower level snapshot.
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.
@benoit the link in the answer's first line covers this.
Read the note there on LVM1 read-only snapshot behaviour and I think you'll have your answer.