[PATCH v4 05/13] dm snapshot: add snapshot-merge target

[Mike Snitzer]

From: Mikulas Patocka <mpatocka@xxxxxxxxxx>

The snapshot-merge target allows a snapshot to be merged back into the
snapshot's origin device.

One expected use of snapshot merging is the rollback of a root
filesystem after system upgrades (e.g.: yum update).  snapshot-merge
enables "system rollback" support for any filesystem that is using the
associated DM devices.

Add snapshot-merge target management to both dm_snapshot_init() and
dm_snapshot_exit().  As an initial place-holder, snapshot-merge is
identical to the snapshot target.

Add "snapshot-merge" to Documentation/device-mapper/snapshot.txt

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
---
 Documentation/device-mapper/snapshot.txt |   52 +++++++++++++++++++++++++++---
 drivers/md/dm-snap.c                     |   48 ++++++++++++++++++++-------
 2 files changed, 83 insertions(+), 17 deletions(-)

diff --git a/Documentation/device-mapper/snapshot.txt b/Documentation/device-mapper/snapshot.txt
index a5009c8..ec6c883 100644
--- a/Documentation/device-mapper/snapshot.txt
+++ b/Documentation/device-mapper/snapshot.txt
@@ -8,13 +8,19 @@ the block device which are also writable without interfering with the
 original content;
 *) To create device "forks", i.e. multiple different versions of the
 same data stream.
+*) To merge a snapshot of a block device back into the snapshot's origin
+device.
 
+In the first two cases, dm copies only the chunks of data that get
+changed and uses a separate copy-on-write (COW) block device for
+storage.
 
-In both cases, dm copies only the chunks of data that get changed and
-uses a separate copy-on-write (COW) block device for storage.
+For snapshot merge the contents of the COW storage are merged back into
+the origin device.
 
 
-There are two dm targets available: snapshot and snapshot-origin.
+There are three dm targets available:
+snapshot, snapshot-origin, and snapshot-merge.
 
 *) snapshot-origin <origin>
 
@@ -40,8 +46,17 @@ The difference is that for transient snapshots less metadata must be
 saved on disk - they can be kept in memory by the kernel.
 
 
-How this is used by LVM2
-========================
+* snapshot-merge <origin> <COW device> <persistent> <chunksize>
+
+takes the same table arguments as the snapshot target except it only
+works with persistent snapshots.  Creates a merging snapshot that takes
+control of the changed chunks stored in the <COW device> of an existing
+snapshot, through a handover procedure, and merges these chunks back
+into the <origin>.
+
+
+How snapshot is used by LVM2
+============================
 When you create the first LVM2 snapshot of a volume, four dm devices are used:
 
 1) a device containing the original mapping table of the source volume;
@@ -72,3 +87,30 @@ brw-------  1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow
 brw-------  1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap
 brw-------  1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base
 
+
+How snapshot-merge is used by LVM2
+==================================
+A merging snapshot assumes the role of the "snapshot-origin" while
+merging.  As such the "snapshot-origin" is replaced with
+"snapshot-merge".  The "-real" device is not changed and the "-cow"
+device is renamed to aid LVM2's cleanup of the merging snapshot after it
+completes.  The "snapshot" that hands over its COW device to the
+"snapshot-merge" is generally deactivated; but if it is left active it
+will simply return I/O errors.
+
+A snapshot will merge into its origin with the following command:
+
+lvconvert --merge volumeGroup/snap
+
+we'll now have this situation:
+
+# dmsetup table|grep volumeGroup
+
+volumeGroup-base-real: 0 2097152 linear 8:19 384
+volumeGroup-base-cow: 0 204800 linear 8:19 2097536
+volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16
+
+# ls -lL /dev/mapper/volumeGroup-*
+brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
+brw-------  1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow
+brw-------  1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base
diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index ad95039..e8a1107 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -1664,6 +1664,21 @@ static struct target_type snapshot_target = {
 	.iterate_devices = snapshot_iterate_devices,
 };
 
+static struct target_type merge_target = {
+	.name    = "snapshot-merge",
+	.version = {1, 9, 0},
+	.module  = THIS_MODULE,
+	.ctr     = snapshot_ctr,
+	.dtr     = snapshot_dtr,
+	.map     = snapshot_map,
+	.end_io  = snapshot_end_io,
+	.postsuspend = snapshot_postsuspend,
+	.preresume  = snapshot_preresume,
+	.resume  = snapshot_resume,
+	.status  = snapshot_status,
+	.iterate_devices = snapshot_iterate_devices,
+};
+
 static int __init dm_snapshot_init(void)
 {
 	int r;
@@ -1675,7 +1690,7 @@ static int __init dm_snapshot_init(void)
 	}
 
 	r = dm_register_target(&snapshot_target);
-	if (r) {
+	if (r < 0) {
 		DMERR("snapshot target register failed %d", r);
 		goto bad_register_snapshot_target;
 	}
@@ -1683,34 +1698,40 @@ static int __init dm_snapshot_init(void)
 	r = dm_register_target(&origin_target);
 	if (r < 0) {
 		DMERR("Origin target register failed %d", r);
-		goto bad1;
+		goto bad_register_origin_target;
+	}
+
+	r = dm_register_target(&merge_target);
+	if (r < 0) {
+		DMERR("Merge target register failed %d", r);
+		goto bad_register_merge_target;
 	}
 
 	r = init_origin_hash();
 	if (r) {
 		DMERR("init_origin_hash failed.");
-		goto bad2;
+		goto bad_origin_hash;
 	}
 
 	exception_cache = KMEM_CACHE(dm_exception, 0);
 	if (!exception_cache) {
 		DMERR("Couldn't create exception cache.");
 		r = -ENOMEM;
-		goto bad3;
+		goto bad_exception_cache;
 	}
 
 	pending_cache = KMEM_CACHE(dm_snap_pending_exception, 0);
 	if (!pending_cache) {
 		DMERR("Couldn't create pending cache.");
 		r = -ENOMEM;
-		goto bad4;
+		goto bad_pending_cache;
 	}
 
 	tracked_chunk_cache = KMEM_CACHE(dm_snap_tracked_chunk, 0);
 	if (!tracked_chunk_cache) {
 		DMERR("Couldn't create cache to track chunks in use.");
 		r = -ENOMEM;
-		goto bad5;
+		goto bad_tracked_chunk_cache;
 	}
 
 	ksnapd = create_singlethread_workqueue("ksnapd");
@@ -1724,19 +1745,21 @@ static int __init dm_snapshot_init(void)
 
 bad_pending_pool:
 	kmem_cache_destroy(tracked_chunk_cache);
-bad5:
+bad_tracked_chunk_cache:
 	kmem_cache_destroy(pending_cache);
-bad4:
+bad_pending_cache:
 	kmem_cache_destroy(exception_cache);
-bad3:
+bad_exception_cache:
 	exit_origin_hash();
-bad2:
+bad_origin_hash:
+	dm_unregister_target(&merge_target);
+bad_register_merge_target:
 	dm_unregister_target(&origin_target);
-bad1:
+bad_register_origin_target:
 	dm_unregister_target(&snapshot_target);
-
 bad_register_snapshot_target:
 	dm_exception_store_exit();
+
 	return r;
 }
 
@@ -1746,6 +1769,7 @@ static void __exit dm_snapshot_exit(void)
 
 	dm_unregister_target(&snapshot_target);
 	dm_unregister_target(&origin_target);
+	dm_unregister_target(&merge_target);
 
 	exit_origin_hash();
 	kmem_cache_destroy(pending_cache);
-- 
1.6.5.2

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