On Mon, 8 Dec 2008, Heinz Mauelshagen wrote: > Am Montag, den 08.12.2008, 09:02 -0500 schrieb Mikulas Patocka: > > > > On Mon, 8 Dec 2008, FUJITA Tomonori wrote: > > > > > I've not closely looked at your dm-multisnapshot but after a quick > > > look, I'm not sure about the number of exception I/Os (except for > > > metadata I/Os) with an origin write. > > > > When writable snapshots will be implemented in my exception store, there > > will be a situation when the code will need to perform more copies. My > > implmentation stores snapshot ID ranges into the btree. For example, if I > > have snapshots 0-10. Someone wrote to snapshot 5. Now someone writes to > > the origin at this chunk. Two copies need to be performed, one for > > snapshots 0-4 and one for snapshots 6-10. > > Don't you just need a copy of the origin for snapshot 5, because it's > being changed and likewise another one for snapshots 0-4,6-10, because > the origin changes, hence snapshot 0-4,6-10 refer to that copy ? > > I.e if any snapshot changes, it gets a private copy to change, either > from the origin unless already changed in which case it receives a copy > from the already existing origin copy. > > Heinz I mean is: If you have snapshots 0-10 and a chunk that is allocated in none of the snapshots, then writing the chunk to the origin allocates one record with range (0-10) in the btree and there is just one copy. But if you have a chunk that is allocated in none of the snapshots, then write this chunk to the snapshot 5, it will copy the chunk and create a record with range (5-5) in the btree. Now, if you write to the origin for that chunk, you need to create two records, one with range (0-4) and the other with range (6-10). That's where multiple copies come from. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
