This is a new implementation of dm-snapshot. The important design differences from the current dm-snapshot are: - It uses one exception store per origin device that is shared by all snapshots. - It doesn't keep the complete exception tables in memory. I took the exception store code of Zumastor (http://zumastor.org/). Zumastor is remote replication software (a local server sends the delta between two snapshots to a remote server, and then the remote server applies the delta in an atomic manner. So the data on the remote server is always consistent). Zumastor snapshot fulfills the above two requirements, but it is implemented in user space. The dm kernel module sends the information of a request to user space and the user space daemon tells the kernel what to do. Zumastor user-space daemon needs to take care about replication so the user-space approach makes sense but I think that the pure user-space approach is an overkill just for snapshot. I prefer to implement snapshot in kernel space (as the current dm-snapshot does). I think that we can add features for remote replication software like Zumastor to it, that is, features to provide user space a delta between two snapshots and apply the delta in an atomic manner (via ioctl or something else). Note that the code is still in a very early stage. There are lots of TODO items: - snapshot deletion support - writable snapshot support - protection for unexpected events (probably journaling) - performance improvement (handling exception cache and format, locking, etc) - better integration with the current snapshot code - improvement on error handling - cleanups - generating a delta between two snapshots - applying a delta to in a atomic manner The patch against 2.6.26 is available at: http://www.kernel.org/pub/linux/kernel/people/tomo/dm-snap/0001-dm-snapshot-dm-snapshot-shared-exception-store.patch Here's an example (/dev/sdb1 as an origin device and /dev/sdg1 as a cow device): - creates the set of an origin and a cow: flax:~# echo 0 `blockdev --getsize /dev/sdb1` snapshot-origin /dev/sdb1 /dev/sdg1 P2 16 |dmsetup create work - no snapshot yet: flax:~# dmsetup status work: 0 125017767 snapshot-origin : no snapshot - creates one snapshot (the id of the snapshot is 0): flax:~# dmsetup message /dev/mapper/work 0 snapshot create 0 - creates one snapshot (the id of the snapshot is 1): flax:~# dmsetup message /dev/mapper/work 0 snapshot create 1 - there are two snapshots (#0 and #1): flax:~# dmsetup status work: 0 125017767 snapshot-origin 0 1 - let's access to the snapshots: flax:~# echo 0 `blockdev --getsize /dev/sdb1` snapshot /dev/sdb1 0|dmsetup create work-snap0 flax:~# echo 0 `blockdev --getsize /dev/sdb1` snapshot /dev/sdb1 1|dmsetup create work-snap1 flax:~# ls /dev/mapper/ control work work-snap0 work-snap1 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
