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+The more-sophisticated device-mapper targets require complex metadata
+that is managed in kernel. In late 2010 we were seeing that various
+different targets were rolling their own data structures, for example:
+- Mikulas Patocka's multisnap implementation
+- Heinz Mauelshagen's thin provisioning target
+- Another btree-based caching target posted to dm-devel
+- Another multi-snapshot target based on a design of Daniel Phillips
+Maintaining these data structures takes a lot of work, so if possible
+we'd like to reduce the number.
+The persistent-data library is an attempt to provide a re-usable
+framework for people who want to store metadata in device-mapper
+targets. It's currently used by the thin-provisioning target and an
+upcoming hierarchical storage target.
+The main documentation is in the header files which can all be found
+under drivers/md/persistent-data.
+The block manager
+This provides access to the data on disk in fixed sized-blocks. There
+is a read/write locking interface to prevent concurrent accesses, and
+keep data that is being used in the cache.
+Clients of persistent-data are unlikely to use this directly.
+The transaction manager
+This restricts access to blocks and enforces copy-on-write semantics.
+The only way you can get hold of a writable block through the
+transaction manager is by shadowing an existing block (ie. doing
+copy-on-write) or allocating a fresh one. Shadowing is elided within
+the same transaction so performance is reasonable. The commit method
+ensures that all data is flushed before it writes the superblock.
+On power failure your metadata will be as it was when last committed.
+The Space Maps
+On-disk data structures that keep track of reference counts of blocks.
+Also acts as the allocator of new blocks. Currently two
+implementations: a simpler one for managing blocks on a different
+device (eg. thinly-provisioned data blocks); and one for managing
+the metadata space. The latter is complicated by the need to store
+its own data within the space it's managing.
+The data structures
+Currently there is only one data structure, a hierarchical btree.
+There are plans to add more. For example, something with an
+array-like interface would see a lot of use.
+The btree is 'hierarchical' in that you can define it to be composed
+of nested btrees, and take multiple keys. For example, the
+thin-provisioning target uses a btree with two levels of nesting.
+The first maps a device id to a mapping tree, and that in turn maps a
+virtual block to a physical block.
+Values stored in the btrees can have arbitrary size. Keys are always
+64bits, although nesting allows you to use multiple keys.