"assume unchanged" git: documentation.

This updates documentation to describe the "assume unchanged"
behaviour.

Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 78 insertions, 0 deletions

diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index c74311dab..0a1b0ad56 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -8,11 +8,14 @@ git-update-index - Modifies the index or directory cache
 
 SYNOPSIS
 --------
+[verse]
 'git-update-index'
 	     [--add] [--remove | --force-remove] [--replace] 
 	     [--refresh [-q] [--unmerged] [--ignore-missing]]
 	     [--cacheinfo <mode> <object> <file>]\*
 	     [--chmod=(+|-)x]
+	     [--assume-unchanged | --no-assume-unchanged]
+	     [--really-refresh]
 	     [--info-only] [--index-info]
 	     [-z] [--stdin]
 	     [--verbose]
@@ -65,6 +68,18 @@ OPTIONS
 --chmod=(+|-)x::
         Set the execute permissions on the updated files.        
 
+--assume-unchanged, --no-assume-unchanged::
+	When these flags are specified, the object name recorded
+	for the paths are not updated.  Instead, these options
+	sets and unsets the "assume unchanged" bit for the
+	paths.  When the "assume unchanged" bit is on, git stops
+	checking the working tree files for possible
+	modifications, so you need to manually unset the bit to
+	tell git when you change the working tree file. This is
+	sometimes helpful when working with a big project on a
+	filesystem that has very slow lstat(2) system call
+	(e.g. cifs).
+
 --info-only::
 	Do not create objects in the object database for all
 	<file> arguments that follow this flag; just insert
@@ -193,6 +208,37 @@ $ git ls-files -s
 ------------
 
 
+Using "assume unchanged" bit
+----------------------------
+
+Many operations in git depend on your filesystem to have an
+efficient `lstat(2)` implementation, so that `st_mtime`
+information for working tree files can be cheaply checked to see
+if the file contents have changed from the version recorded in
+the index file.  Unfortunately, some filesystems have
+inefficient `lstat(2)`.  If your filesystem is one of them, you
+can set "assume unchanged" bit to paths you have not changed to
+cause git not to do this check.  Note that setting this bit on a
+path does not mean git will check the contents of the file to
+see if it has changed -- it makes git to omit any checking and
+assume it has *not* changed.  When you make changes to working
+tree files, you have to explicitly tell git about it by dropping
+"assume unchanged" bit, either before or after you modify them.
+
+In order to set "assume unchanged" bit, use `--assume-unchanged`
+option.  To unset, use `--no-assume-unchanged`.
+
+The command looks at `core.ignorestat` configuration variable.  When
+this is true, paths updated with `git-update-index paths...` and
+paths updated with other git commands that update both index and
+working tree (e.g. `git-apply --index`, `git-checkout-index -u`,
+and `git-read-tree -u`) are automatically marked as "assume
+unchanged".  Note that "assume unchanged" bit is *not* set if
+`git-update-index --refresh` finds the working tree file matches
+the index (use `git-update-index --really-refresh` if you want
+to mark them as "assume unchanged").
+
+
 Examples
 --------
 To update and refresh only the files already checked out:
@@ -201,6 +247,35 @@ To update and refresh only the files already checked out:
 $ git-checkout-index -n -f -a && git-update-index --ignore-missing --refresh
 ----------------
 
+On an inefficient filesystem with `core.ignorestat` set:
+
+------------
+$ git update-index --really-refresh <1>
+$ git update-index --no-assume-unchanged foo.c <2>
+$ git diff --name-only <3>
+$ edit foo.c
+$ git diff --name-only <4>
+M foo.c
+$ git update-index foo.c <5>
+$ git diff --name-only <6>
+$ edit foo.c
+$ git diff --name-only <7>
+$ git update-index --no-assume-unchanged foo.c <8>
+$ git diff --name-only <9>
+M foo.c
+
+<1> forces lstat(2) to set "assume unchanged" bits for paths
+    that match index.
+<2> mark the path to be edited.
+<3> this does lstat(2) and finds index matches the path.
+<4> this does lstat(2) and finds index does not match the path.
+<5> registering the new version to index sets "assume unchanged" bit.
+<6> and it is assumed unchanged.
+<7> even after you edit it.
+<8> you can tell about the change after the fact.
+<9> now it checks with lstat(2) and finds it has been changed.
+------------
+
 
 Configuration
 -------------
@@ -213,6 +288,9 @@ in the index and the file mode on the filesystem if they differ only on
 executable bit.   On such an unfortunate filesystem, you may
 need to use `git-update-index --chmod=`.
 
+The command looks at `core.ignorestat` configuration variable.  See
+'Using "assume unchanged" bit' section above.
+
 
 See Also
 --------