[PATCH] Device-mapper: snapshots

Add snapshot target
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/md/Kconfig

@@ -180,5 +180,11 @@ config DM_CRYPT
 
 	  If unsure, say N.
 
+config DM_SNAPSHOT
+       tristate "Snapshot target (EXPERIMENTAL)"
+       depends on BLK_DEV_DM && EXPERIMENTAL
+       ---help---
+         Allow volume managers to take writeable snapshots of a device.
+
 endmenu
 

drivers/md/Makefile

@@ -4,6 +4,7 @@
 
 dm-mod-objs	:= dm.o dm-table.o dm-target.o dm-linear.o dm-stripe.o \
 		   dm-ioctl.o dm-io.o kcopyd.o
+dm-snapshot-objs := dm-snap.o dm-exception-store.o
 raid6-objs	:= raid6main.o raid6algos.o raid6recov.o raid6tables.o \
 		   raid6int1.o raid6int2.o raid6int4.o \
 		   raid6int8.o raid6int16.o raid6int32.o \
@@ -24,6 +25,7 @@ obj-$(CONFIG_MD_MULTIPATH)	+= multipath.o
 obj-$(CONFIG_BLK_DEV_MD)	+= md.o
 obj-$(CONFIG_BLK_DEV_DM)	+= dm-mod.o
 obj-$(CONFIG_DM_CRYPT)		+= dm-crypt.o
+obj-$(CONFIG_DM_SNAPSHOT)	+= dm-snapshot.o
 
 quiet_cmd_unroll = UNROLL  $@
       cmd_unroll = $(PERL) $(srctree)/$(src)/unroll.pl $(UNROLL) \

drivers/md/dm-snap.h

+/*
+ * dm-snapshot.c
+ *
+ * Copyright (C) 2001-2002 Sistina Software (UK) Limited.
+ *
+ * This file is released under the GPL.
+ */
+
+#ifndef DM_SNAPSHOT_H
+#define DM_SNAPSHOT_H
+
+#include "dm.h"
+#include <linux/blkdev.h>
+
+struct exception_table {
+	uint32_t hash_mask;
+	struct list_head *table;
+};
+
+/*
+ * The snapshot code deals with largish chunks of the disk at a
+ * time. Typically 64k - 256k.
+ */
+/* FIXME: can we get away with limiting these to a uint32_t ? */
+typedef sector_t chunk_t;
+
+/*
+ * An exception is used where an old chunk of data has been
+ * replaced by a new one.
+ */
+struct exception {
+	struct list_head hash_list;
+
+	chunk_t old_chunk;
+	chunk_t new_chunk;
+};
+
+/*
+ * Abstraction to handle the meta/layout of exception stores (the
+ * COW device).
+ */
+struct exception_store {
+
+	/*
+	 * Destroys this object when you've finished with it.
+	 */
+	void (*destroy) (struct exception_store *store);
+
+	/*
+	 * The target shouldn't read the COW device until this is
+	 * called.
+	 */
+	int (*read_metadata) (struct exception_store *store);
+
+	/*
+	 * Find somewhere to store the next exception.
+	 */
+	int (*prepare_exception) (struct exception_store *store,
+				  struct exception *e);
+
+	/*
+	 * Update the metadata with this exception.
+	 */
+	void (*commit_exception) (struct exception_store *store,
+				  struct exception *e,
+				  void (*callback) (void *, int success),
+				  void *callback_context);
+
+	/*
+	 * The snapshot is invalid, note this in the metadata.
+	 */
+	void (*drop_snapshot) (struct exception_store *store);
+
+	/*
+	 * Return how full the snapshot is.
+	 */
+	void (*fraction_full) (struct exception_store *store,
+			       sector_t *numerator,
+			       sector_t *denominator);
+
+	struct dm_snapshot *snap;
+	void *context;
+};
+
+struct dm_snapshot {
+	struct rw_semaphore lock;
+	struct dm_table *table;
+
+	struct dm_dev *origin;
+	struct dm_dev *cow;
+
+	/* List of snapshots per Origin */
+	struct list_head list;
+
+	/* Size of data blocks saved - must be a power of 2 */
+	chunk_t chunk_size;
+	chunk_t chunk_mask;
+	chunk_t chunk_shift;
+
+	/* You can't use a snapshot if this is 0 (e.g. if full) */
+	int valid;
+	int have_metadata;
+
+	/* Used for display of table */
+	char type;
+
+	/* The last percentage we notified */
+	int last_percent;
+
+	struct exception_table pending;
+	struct exception_table complete;
+
+	/* The on disk metadata handler */
+	struct exception_store store;
+
+	struct kcopyd_client *kcopyd_client;
+};
+
+/*
+ * Used by the exception stores to load exceptions hen
+ * initialising.
+ */
+int dm_add_exception(struct dm_snapshot *s, chunk_t old, chunk_t new);
+
+/*
+ * Constructor and destructor for the default persistent
+ * store.
+ */
+int dm_create_persistent(struct exception_store *store, uint32_t chunk_size);
+
+int dm_create_transient(struct exception_store *store,
+			struct dm_snapshot *s, int blocksize);
+
+/*
+ * Return the number of sectors in the device.
+ */
+static inline sector_t get_dev_size(struct block_device *bdev)
+{
+	return bdev->bd_inode->i_size >> SECTOR_SHIFT;
+}
+
+static inline chunk_t sector_to_chunk(struct dm_snapshot *s, sector_t sector)
+{
+	return (sector & ~s->chunk_mask) >> s->chunk_shift;
+}
+
+static inline sector_t chunk_to_sector(struct dm_snapshot *s, chunk_t chunk)
+{
+	return chunk << s->chunk_shift;
+}
+
+static inline int bdev_equal(struct block_device *lhs, struct block_device *rhs)
+{
+	/*
+	 * There is only ever one instance of a particular block
+	 * device so we can compare pointers safely.
+	 */
+	return lhs == rhs;
+}
+
+#endif