1 ===============
2 Persistent data
3 ===============
4
5 Introduction
6 ============
7
8 The more-sophisticated device-mapper targets r
9 that is managed in kernel. In late 2010 we we
10 different targets were rolling their own data
11
12 - Mikulas Patocka's multisnap implementation
13 - Heinz Mauelshagen's thin provisioning target
14 - Another btree-based caching target posted to
15 - Another multi-snapshot target based on a des
16
17 Maintaining these data structures takes a lot
18 we'd like to reduce the number.
19
20 The persistent-data library is an attempt to p
21 framework for people who want to store metadat
22 targets. It's currently used by the thin-prov
23 upcoming hierarchical storage target.
24
25 Overview
26 ========
27
28 The main documentation is in the header files
29 under drivers/md/persistent-data.
30
31 The block manager
32 -----------------
33
34 dm-block-manager.[hc]
35
36 This provides access to the data on disk in fi
37 is a read/write locking interface to prevent c
38 keep data that is being used in the cache.
39
40 Clients of persistent-data are unlikely to use
41
42 The transaction manager
43 -----------------------
44
45 dm-transaction-manager.[hc]
46
47 This restricts access to blocks and enforces c
48 The only way you can get hold of a writable bl
49 transaction manager is by shadowing an existin
50 copy-on-write) or allocating a fresh one. Sha
51 the same transaction so performance is reasona
52 ensures that all data is flushed before it wri
53 On power failure your metadata will be as it w
54
55 The Space Maps
56 --------------
57
58 dm-space-map.h
59 dm-space-map-metadata.[hc]
60 dm-space-map-disk.[hc]
61
62 On-disk data structures that keep track of ref
63 Also acts as the allocator of new blocks. Cur
64 implementations: a simpler one for managing bl
65 device (eg. thinly-provisioned data blocks); a
66 the metadata space. The latter is complicated
67 its own data within the space it's managing.
68
69 The data structures
70 -------------------
71
72 dm-btree.[hc]
73 dm-btree-remove.c
74 dm-btree-spine.c
75 dm-btree-internal.h
76
77 Currently there is only one data structure, a
78 There are plans to add more. For example, som
79 array-like interface would see a lot of use.
80
81 The btree is 'hierarchical' in that you can de
82 of nested btrees, and take multiple keys. For
83 thin-provisioning target uses a btree with two
84 The first maps a device id to a mapping tree,
85 virtual block to a physical block.
86
87 Values stored in the btrees can have arbitrary
88 64bits, although nesting allows you to use mul
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