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>
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. This target assumes the role of the
+"snapshot-origin" target and must not be loaded if the "snapshot-origin"
+is still present for <origin>.
+
+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>. Once merging
+has started (in the background) the <origin> may be opened and the merge
+will continue while I/O is flowing to it. Changes to the <origin> are
+deferred until the merging snapshot's corresponding chunk(s) have been
+merged. Once merging has started the snapshot device, associated with
+the "snapshot" target, will return -EIO when accessed.
+
+
+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;
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 <origin name>-cow 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 deactivated (unless using lvchange
+--refresh); 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
#define DM_MSG_PREFIX "snapshots"
+static const char dm_snapshot_merge_target_name[] = "snapshot-merge";
+
+#define dm_target_is_snapshot_merge(ti) \
+ ((ti)->type->name == dm_snapshot_merge_target_name)
+
/*
* The percentage increment we will wake up users at
*/
.iterate_devices = snapshot_iterate_devices,
};
+static struct target_type merge_target = {
+ .name = dm_snapshot_merge_target_name,
+ .version = {1, 0, 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;
}
r = dm_register_target(&snapshot_target);
- if (r) {
+ if (r < 0) {
DMERR("snapshot target register failed %d", r);
goto bad_register_snapshot_target;
}
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");
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;
}
dm_unregister_target(&snapshot_target);
dm_unregister_target(&origin_target);
+ dm_unregister_target(&merge_target);
exit_origin_hash();
kmem_cache_destroy(pending_cache);
git.samba.org / sfrench / cifs-2.6.git / commitdiff