@@ -23,6 +23,52 @@
#include "delalloc-space.h"
#include "block-group.h"
+/*
+ * Introduction for btrfs relocation.
+ *
+ * [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, or just
+ * balance routine to free some block groups.
+ *
+ * In short, relocation wants to do:
+ * 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 bg RO
+ * So that new extents won't be allocated from the target bg.
+ *
+ * 2.1 Record each extent in the target bg
+ * 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 bg will get its reloc
+ * tree built.
+ *
+ * 2.3 Swap source tree with its corresponding reloc tree
+ * So that each involved tree only refers to new extents after swap.
+ *
+ * 3. Cleanup reloc trees and data reloc tree.
+ * As old extents in the target bg is still referred by reloc trees,
+ * we need to clean them up before really freeing the target bg.
+ *
+ * The main complexity is in steps 2.2 and 2.3.
+ *
+ * The core entrance for relocation is relocate_block_group() function.
+ */
/*
* backref_node, mapping_node and tree_block start with this
*/
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. For now the introduction should save reader some time before digging into the rabbit hole. Signed-off-by: Qu Wenruo <wqu@suse.com> --- Changelog: v2: - New line after section title to improve readability - Don't mention "to relocate some extents" part for the objective. As that only happens for error case. - Grammar fix. --- fs/btrfs/relocation.c | 46 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 46 insertions(+)