Documentation/git-update-index: explain splitIndex.*

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2795,7 +2795,7 @@ splitIndex.sharedIndexExpire::
 	The default value is "2.weeks.ago".
 	Note that a shared index file is considered modified (for the
 	purpose of expiration) each time a new split-index file is
-	created based on it.
+	either created based on it or read from it.
 	See linkgit:git-update-index[1].
 
 status.relativePaths::

Documentation/git-update-index.txt

@@ -163,14 +163,10 @@ may not support it yet.
 
 --split-index::
 --no-split-index::
-	Enable or disable split index mode. If enabled, the index is
-	split into two files, $GIT_DIR/index and $GIT_DIR/sharedindex.<SHA-1>.
-	Changes are accumulated in $GIT_DIR/index while the shared
-	index file contains all index entries stays unchanged. If
-	split-index mode is already enabled and `--split-index` is
-	given again, all changes in $GIT_DIR/index are pushed back to
-	the shared index file. This mode is designed for very large
-	indexes that take a significant amount of time to read or write.
+	Enable or disable split index mode. If split-index mode is
+	already enabled and `--split-index` is given again, all
+	changes in $GIT_DIR/index are pushed back to the shared index
+	file.
 +
 These options take effect whatever the value of the `core.splitIndex`
 configuration variable (see linkgit:git-config[1]). But a warning is
@@ -394,6 +390,31 @@ Although this bit looks similar to assume-unchanged bit, its goal is
 different from assume-unchanged bit's. Skip-worktree also takes
 precedence over assume-unchanged bit when both are set.
 
+Split index
+-----------
+
+This mode is designed for repositories with very large indexes, and
+aims at reducing the time it takes to repeatedly write these indexes.
+
+In this mode, the index is split into two files, $GIT_DIR/index and
+$GIT_DIR/sharedindex.<SHA-1>. Changes are accumulated in
+$GIT_DIR/index, the split index, while the shared index file contains
+all index entries and stays unchanged.
+
+All changes in the split index are pushed back to the shared index
+file when the number of entries in the split index reaches a level
+specified by the splitIndex.maxPercentChange config variable (see
+linkgit:git-config[1]).
+
+Each time a new shared index file is created, the old shared index
+files are deleted if their modification time is older than what is
+specified by the splitIndex.sharedIndexExpire config variable (see
+linkgit:git-config[1]).
+
+To avoid deleting a shared index file that is still used, its
+modification time is updated to the current time everytime a new split
+index based on the shared index file is either created or read from.
+
 Untracked cache
 ---------------