From cc8394d86f045b86ff303d3c9e4ce47d97148951 Mon Sep 17 00:00:00 2001 From: Joe Thornber Date: Sun, 3 Jun 2012 00:30:01 +0100 Subject: dm thin: provide userspace access to pool metadata This patch implements two new messages that can be sent to the thin pool target allowing it to take a snapshot of the _metadata_. This, read-only snapshot can be accessed by userland, concurrently with the live target. Only one metadata snapshot can be held at a time. The pool's status line will give the block location for the current msnap. Since version 0.1.5 of the userland thin provisioning tools, the thin_dump program displays the msnap as follows: thin_dump -m Available here: https://github.com/jthornber/thin-provisioning-tools Now that userland can access the metadata we can do various things that have traditionally been kernel side tasks: i) Incremental backups. By using metadata snapshots we can work out what blocks have changed over time. Combined with data snapshots we can ensure the data doesn't change while we back it up. A short proof of concept script can be found here: https://github.com/jthornber/thinp-test-suite/blob/master/incremental_backup_example.rb ii) Migration of thin devices from one pool to another. iii) Merging snapshots back into an external origin. iv) Asyncronous replication. Signed-off-by: Joe Thornber Signed-off-by: Alasdair G Kergon --- Documentation/device-mapper/thin-provisioning.txt | 11 +++++++++++ 1 file changed, 11 insertions(+) (limited to 'Documentation') diff --git a/Documentation/device-mapper/thin-provisioning.txt b/Documentation/device-mapper/thin-provisioning.txt index 3370bc4d7b98..f5cfc62b7ad3 100644 --- a/Documentation/device-mapper/thin-provisioning.txt +++ b/Documentation/device-mapper/thin-provisioning.txt @@ -287,6 +287,17 @@ iii) Messages the current transaction id is when you change it with this compare-and-swap message. + reserve_metadata_snap + + Reserve a copy of the data mapping btree for use by userland. + This allows userland to inspect the mappings as they were when + this message was executed. Use the pool's status command to + get the root block associated with the metadata snapshot. + + release_metadata_snap + + Release a previously reserved copy of the data mapping btree. + 'thin' target ------------- -- cgit v1.2.3-59-g8ed1b