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