read-cache: split-index mode

This split-index mode is designed to keep write cost proportional to
the number of changes the user has made, not the size of the work
tree. (Read cost is another matter, to be dealt separately.)

This mode stores index info in a pair of $GIT_DIR/index and
$GIT_DIR/sharedindex.<SHA-1>. sharedindex is large and unchanged over
time while "index" is smaller and updated often. Format details are in
index-format.txt, although not everything is implemented in this
patch.

Shared indexes are not automatically removed, because it's unclear if
the shared index is needed by any (even temporary) indexes by just
looking at it. After a while you'll collect stale shared indexes. The
good news is one shared index is useable for long, until
$GIT_DIR/index becomes too big and sluggish that the new shared index
must be created.

The safest way to clean shared indexes is to turn off split index
mode, so shared files are all garbage, delete them all, then turn on
split index mode again.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/cache.h b/cache.h
index 41cdcd0..47fe374 100644
--- a/cache.h
+++ b/cache.h
@@ -135,6 +135,7 @@ struct cache_entry {
 	unsigned int ce_mode;
 	unsigned int ce_flags;
 	unsigned int ce_namelen;
+	unsigned int index;	/* for link extension */
 	unsigned char sha1[20];
 	char name[FLEX_ARRAY]; /* more */
 };
@@ -275,12 +276,14 @@ static inline unsigned int canon_mode(unsigned int mode)
 #define RESOLVE_UNDO_CHANGED	(1 << 4)
 #define CACHE_TREE_CHANGED	(1 << 5)
 
+struct split_index;
 struct index_state {
 	struct cache_entry **cache;
 	unsigned int version;
 	unsigned int cache_nr, cache_alloc, cache_changed;
 	struct string_list *resolve_undo;
 	struct cache_tree *cache_tree;
+	struct split_index *split_index;
 	struct cache_time timestamp;
 	unsigned name_hash_initialized : 1,
 		 initialized : 1;