mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-11-30 14:16:41 +07:00
40e47125e6
Signed-off-by: Masanari Iida <standby24x7@gmail.com> Acked-by: Randy Dunlap <rdunlap@xenotime.net> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
85 lines
2.9 KiB
Plaintext
85 lines
2.9 KiB
Plaintext
Introduction
|
|
============
|
|
|
|
The more-sophisticated device-mapper targets require complex metadata
|
|
that is managed in kernel. In late 2010 we were seeing that various
|
|
different targets were rolling their own data structures, for example:
|
|
|
|
- Mikulas Patocka's multisnap implementation
|
|
- Heinz Mauelshagen's thin provisioning target
|
|
- Another btree-based caching target posted to dm-devel
|
|
- Another multi-snapshot target based on a design of Daniel Phillips
|
|
|
|
Maintaining these data structures takes a lot of work, so if possible
|
|
we'd like to reduce the number.
|
|
|
|
The persistent-data library is an attempt to provide a re-usable
|
|
framework for people who want to store metadata in device-mapper
|
|
targets. It's currently used by the thin-provisioning target and an
|
|
upcoming hierarchical storage target.
|
|
|
|
Overview
|
|
========
|
|
|
|
The main documentation is in the header files which can all be found
|
|
under drivers/md/persistent-data.
|
|
|
|
The block manager
|
|
-----------------
|
|
|
|
dm-block-manager.[hc]
|
|
|
|
This provides access to the data on disk in fixed sized-blocks. There
|
|
is a read/write locking interface to prevent concurrent accesses, and
|
|
keep data that is being used in the cache.
|
|
|
|
Clients of persistent-data are unlikely to use this directly.
|
|
|
|
The transaction manager
|
|
-----------------------
|
|
|
|
dm-transaction-manager.[hc]
|
|
|
|
This restricts access to blocks and enforces copy-on-write semantics.
|
|
The only way you can get hold of a writable block through the
|
|
transaction manager is by shadowing an existing block (ie. doing
|
|
copy-on-write) or allocating a fresh one. Shadowing is elided within
|
|
the same transaction so performance is reasonable. The commit method
|
|
ensures that all data is flushed before it writes the superblock.
|
|
On power failure your metadata will be as it was when last committed.
|
|
|
|
The Space Maps
|
|
--------------
|
|
|
|
dm-space-map.h
|
|
dm-space-map-metadata.[hc]
|
|
dm-space-map-disk.[hc]
|
|
|
|
On-disk data structures that keep track of reference counts of blocks.
|
|
Also acts as the allocator of new blocks. Currently two
|
|
implementations: a simpler one for managing blocks on a different
|
|
device (eg. thinly-provisioned data blocks); and one for managing
|
|
the metadata space. The latter is complicated by the need to store
|
|
its own data within the space it's managing.
|
|
|
|
The data structures
|
|
-------------------
|
|
|
|
dm-btree.[hc]
|
|
dm-btree-remove.c
|
|
dm-btree-spine.c
|
|
dm-btree-internal.h
|
|
|
|
Currently there is only one data structure, a hierarchical btree.
|
|
There are plans to add more. For example, something with an
|
|
array-like interface would see a lot of use.
|
|
|
|
The btree is 'hierarchical' in that you can define it to be composed
|
|
of nested btrees, and take multiple keys. For example, the
|
|
thin-provisioning target uses a btree with two levels of nesting.
|
|
The first maps a device id to a mapping tree, and that in turn maps a
|
|
virtual block to a physical block.
|
|
|
|
Values stored in the btrees can have arbitrary size. Keys are always
|
|
64bits, although nesting allows you to use multiple keys.
|