1=============== 2Persistent data 3=============== 4 5Introduction 6============ 7 8The more-sophisticated device-mapper targets require complex metadata 9that is managed in kernel. In late 2010 we were seeing that various 10different targets were rolling their own data structures, for example: 11 12- Mikulas Patocka's multisnap implementation 13- Heinz Mauelshagen's thin provisioning target 14- Another btree-based caching target posted to dm-devel 15- Another multi-snapshot target based on a design of Daniel Phillips 16 17Maintaining these data structures takes a lot of work, so if possible 18we'd like to reduce the number. 19 20The persistent-data library is an attempt to provide a re-usable 21framework for people who want to store metadata in device-mapper 22targets. It's currently used by the thin-provisioning target and an 23upcoming hierarchical storage target. 24 25Overview 26======== 27 28The main documentation is in the header files which can all be found 29under drivers/md/persistent-data. 30 31The block manager 32----------------- 33 34dm-block-manager.[hc] 35 36This provides access to the data on disk in fixed sized-blocks. There 37is a read/write locking interface to prevent concurrent accesses, and 38keep data that is being used in the cache. 39 40Clients of persistent-data are unlikely to use this directly. 41 42The transaction manager 43----------------------- 44 45dm-transaction-manager.[hc] 46 47This restricts access to blocks and enforces copy-on-write semantics. 48The only way you can get hold of a writable block through the 49transaction manager is by shadowing an existing block (ie. doing 50copy-on-write) or allocating a fresh one. Shadowing is elided within 51the same transaction so performance is reasonable. The commit method 52ensures that all data is flushed before it writes the superblock. 53On power failure your metadata will be as it was when last committed. 54 55The Space Maps 56-------------- 57 58dm-space-map.h 59dm-space-map-metadata.[hc] 60dm-space-map-disk.[hc] 61 62On-disk data structures that keep track of reference counts of blocks. 63Also acts as the allocator of new blocks. Currently two 64implementations: a simpler one for managing blocks on a different 65device (eg. thinly-provisioned data blocks); and one for managing 66the metadata space. The latter is complicated by the need to store 67its own data within the space it's managing. 68 69The data structures 70------------------- 71 72dm-btree.[hc] 73dm-btree-remove.c 74dm-btree-spine.c 75dm-btree-internal.h 76 77Currently there is only one data structure, a hierarchical btree. 78There are plans to add more. For example, something with an 79array-like interface would see a lot of use. 80 81The btree is 'hierarchical' in that you can define it to be composed 82of nested btrees, and take multiple keys. For example, the 83thin-provisioning target uses a btree with two levels of nesting. 84The first maps a device id to a mapping tree, and that in turn maps a 85virtual block to a physical block. 86 87Values stored in the btrees can have arbitrary size. Keys are always 8864bits, although nesting allows you to use multiple keys. 89