Commit 0c891389 authored by Qu Wenruo's avatar Qu Wenruo Committed by David Sterba

btrfs: relocation: Add introduction of how relocation works

Relocation is one of the most complex part of btrfs, while it's also the
foundation stone for online resizing, profile converting.

For such a complex facility, we should at least have some introduction
to it.

This patch will add an basic introduction at pretty a high level,
explaining:

- What relocation does
- How relocation is done
  Only mentioning how data reloc tree and reloc tree are involved in the
  operation.
  No details like the backref cache, or the data reloc tree contents.
- Which function to refer.

More detailed comments will be added for reloc tree creation, data reloc
tree creation and backref cache.
Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 42836cf4
...@@ -23,6 +23,53 @@ ...@@ -23,6 +23,53 @@
#include "delalloc-space.h" #include "delalloc-space.h"
#include "block-group.h" #include "block-group.h"
/*
* Relocation overview
*
* [What does relocation do]
*
* The objective of relocation is to relocate all extents of the target block
* group to other block groups.
* This is utilized by resize (shrink only), profile converting, compacting
* space, or balance routine to spread chunks over devices.
*
* Before | After
* ------------------------------------------------------------------
* BG A: 10 data extents | BG A: deleted
* BG B: 2 data extents | BG B: 10 data extents (2 old + 8 relocated)
* BG C: 1 extents | BG C: 3 data extents (1 old + 2 relocated)
*
* [How does relocation work]
*
* 1. Mark the target block group read-only
* New extents won't be allocated from the target block group.
*
* 2.1 Record each extent in the target block group
* To build a proper map of extents to be relocated.
*
* 2.2 Build data reloc tree and reloc trees
* Data reloc tree will contain an inode, recording all newly relocated
* data extents.
* There will be only one data reloc tree for one data block group.
*
* Reloc tree will be a special snapshot of its source tree, containing
* relocated tree blocks.
* Each tree referring to a tree block in target block group will get its
* reloc tree built.
*
* 2.3 Swap source tree with its corresponding reloc tree
* Each involved tree only refers to new extents after swap.
*
* 3. Cleanup reloc trees and data reloc tree.
* As old extents in the target block group are still referenced by reloc
* trees, we need to clean them up before really freeing the target block
* group.
*
* The main complexity is in steps 2.2 and 2.3.
*
* The entry point of relocation is relocate_block_group() function.
*/
/* /*
* backref_node, mapping_node and tree_block start with this * backref_node, mapping_node and tree_block start with this
*/ */
......
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