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 <msnap root> <metadata dev>

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 <ejt@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 40 insertions, 2 deletions

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index db1b041ce975..37fdaf81bd1f 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -2284,6 +2284,36 @@ static int process_set_transaction_id_mesg(unsigned argc, char **argv, struct po
         return 0;
 }
 
+static int process_reserve_metadata_snap_mesg(unsigned argc, char **argv, struct pool *pool)
+{
+        int r;
+
+        r = check_arg_count(argc, 1);
+        if (r)
+                return r;
+
+        r = dm_pool_reserve_metadata_snap(pool->pmd);
+        if (r)
+                DMWARN("reserve_metadata_snap message failed.");
+
+        return r;
+}
+
+static int process_release_metadata_snap_mesg(unsigned argc, char **argv, struct pool *pool)
+{
+        int r;
+
+        r = check_arg_count(argc, 1);
+        if (r)
+                return r;
+
+        r = dm_pool_release_metadata_snap(pool->pmd);
+        if (r)
+                DMWARN("release_metadata_snap message failed.");
+
+        return r;
+}
+
 /*
  * Messages supported:
  *   create_thin        <dev_id>
@@ -2291,6 +2321,8 @@ static int process_set_transaction_id_mesg(unsigned argc, char **argv, struct po
  *   delete             <dev_id>
  *   trim               <dev_id> <new_size_in_sectors>
  *   set_transaction_id <current_trans_id> <new_trans_id>
+ *   reserve_metadata_snap
+ *   release_metadata_snap
  */
 static int pool_message(struct dm_target *ti, unsigned argc, char **argv)
 {
@@ -2310,6 +2342,12 @@ static int pool_message(struct dm_target *ti, unsigned argc, char **argv)
         else if (!strcasecmp(argv[0], "set_transaction_id"))
                 r = process_set_transaction_id_mesg(argc, argv, pool);
 
+        else if (!strcasecmp(argv[0], "reserve_metadata_snap"))
+                r = process_reserve_metadata_snap_mesg(argc, argv, pool);
+
+        else if (!strcasecmp(argv[0], "release_metadata_snap"))
+                r = process_release_metadata_snap_mesg(argc, argv, pool);
+
         else
                 DMWARN("Unrecognised thin pool target message received: %s", argv[0]);
 
@@ -2369,7 +2407,7 @@ static int pool_status(struct dm_target *ti, status_type_t type,
                 if (r)
                         return r;
 
-                r = dm_pool_get_held_metadata_root(pool->pmd, &held_root);
+                r = dm_pool_get_metadata_snap(pool->pmd, &held_root);
                 if (r)
                         return r;
 
@@ -2465,7 +2503,7 @@ static struct target_type pool_target = {
         .name = "thin-pool",
         .features = DM_TARGET_SINGLETON | DM_TARGET_ALWAYS_WRITEABLE |
                     DM_TARGET_IMMUTABLE,
-        .version = {1, 1, 0},
+        .version = {1, 2, 0},
         .module = THIS_MODULE,
         .ctr = pool_ctr,
         .dtr = pool_dtr,