Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 Introduction 2 ============ 3 4 The more-sophisticated device-mapper targets require complex metadata 5 that is managed in kernel. In late 2010 we were seeing that various 6 different 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 13 Maintaining these data structures takes a lot of work, so if possible 14 we'd like to reduce the number. 15 16 The persistent-data library is an attempt to provide a re-usable 17 framework for people who want to store metadata in device-mapper 18 targets. It's currently used by the thin-provisioning target and an 19 upcoming hierarchical storage target. 20 21 Overview 22 ======== 23 24 The main documentation is in the header files which can all be found 25 under drivers/md/persistent-data. 26 27 The block manager 28 ----------------- 29 30 dm-block-manager.[hc] 31 32 This provides access to the data on disk in fixed sized-blocks. There 33 is a read/write locking interface to prevent concurrent accesses, and 34 keep data that is being used in the cache. 35 36 Clients of persistent-data are unlikely to use this directly. 37 38 The transaction manager 39 ----------------------- 40 41 dm-transaction-manager.[hc] 42 43 This restricts access to blocks and enforces copy-on-write semantics. 44 The only way you can get hold of a writable block through the 45 transaction manager is by shadowing an existing block (ie. doing 46 copy-on-write) or allocating a fresh one. Shadowing is elided within 47 the same transaction so performance is reasonable. The commit method 48 ensures that all data is flushed before it writes the superblock. 49 On power failure your metadata will be as it was when last committed. 50 51 The Space Maps 52 -------------- 53 54 dm-space-map.h 55 dm-space-map-metadata.[hc] 56 dm-space-map-disk.[hc] 57 58 On-disk data structures that keep track of reference counts of blocks. 59 Also acts as the allocator of new blocks. Currently two 60 implementations: a simpler one for managing blocks on a different 61 device (eg. thinly-provisioned data blocks); and one for managing 62 the metadata space. The latter is complicated by the need to store 63 its own data within the space it's managing. 64 65 The data structures 66 ------------------- 67 68 dm-btree.[hc] 69 dm-btree-remove.c 70 dm-btree-spine.c 71 dm-btree-internal.h 72 73 Currently there is only one data structure, a hierarchical btree. 74 There are plans to add more. For example, something with an 75 array-like interface would see a lot of use. 76 77 The btree is 'hierarchical' in that you can define it to be composed 78 of nested btrees, and take multiple keys. For example, the 79 thin-provisioning target uses a btree with two levels of nesting. 80 The first maps a device id to a mapping tree, and that in turn maps a 81 virtual block to a physical block. 82 83 Values stored in the btrees can have arbitrary size. Keys are always 84 64bits, although nesting allows you to use multiple keys.