1Introduction 2============ 3 4The more-sophisticated device-mapper targets require complex metadata 5that is managed in kernel. In late 2010 we were seeing that various 6different targets were rolling their own data structures, for example: 7 8- Mikulas Patocka's multisnap implementation 9- Heinz Mauelshagen's thin provisioning target 10- Another btree-based caching target posted to dm-devel 11- Another multi-snapshot target based on a design of Daniel Phillips 12 13Maintaining these data structures takes a lot of work, so if possible 14we'd like to reduce the number. 15 16The persistent-data library is an attempt to provide a re-usable 17framework for people who want to store metadata in device-mapper 18targets. It's currently used by the thin-provisioning target and an 19upcoming hierarchical storage target. 20 21Overview 22======== 23 24The main documentation is in the header files which can all be found 25under drivers/md/persistent-data. 26 27The block manager 28----------------- 29 30dm-block-manager.[hc] 31 32This provides access to the data on disk in fixed sized-blocks. There 33is a read/write locking interface to prevent concurrent accesses, and 34keep data that is being used in the cache. 35 36Clients of persistent-data are unlikely to use this directly. 37 38The transaction manager 39----------------------- 40 41dm-transaction-manager.[hc] 42 43This restricts access to blocks and enforces copy-on-write semantics. 44The only way you can get hold of a writable block through the 45transaction manager is by shadowing an existing block (ie. doing 46copy-on-write) or allocating a fresh one. Shadowing is elided within 47the same transaction so performance is reasonable. The commit method 48ensures that all data is flushed before it writes the superblock. 49On power failure your metadata will be as it was when last committed. 50 51The Space Maps 52-------------- 53 54dm-space-map.h 55dm-space-map-metadata.[hc] 56dm-space-map-disk.[hc] 57 58On-disk data structures that keep track of reference counts of blocks. 59Also acts as the allocator of new blocks. Currently two 60implementations: a simpler one for managing blocks on a different 61device (eg. thinly-provisioned data blocks); and one for managing 62the metadata space. The latter is complicated by the need to store 63its own data within the space it's managing. 64 65The data structures 66------------------- 67 68dm-btree.[hc] 69dm-btree-remove.c 70dm-btree-spine.c 71dm-btree-internal.h 72 73Currently there is only one data structure, a hierarchical btree. 74There are plans to add more. For example, something with an 75array-like interface would see a lot of use. 76 77The btree is 'hierarchical' in that you can define it to be composed 78of nested btrees, and take multiple keys. For example, the 79thin-provisioning target uses a btree with two levels of nesting. 80The first maps a device id to a mapping tree, and that in turn maps a 81virtual block to a physical block. 82 83Values stored in the btrees can have arbitrary size. Keys are always 8464bits, although nesting allows you to use multiple keys. 85