Add documents/comments for dm-queue-length.

Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
---
 Documentation/device-mapper/dm-queue-length.txt |   39 ++++++++++++++++++++++++
 drivers/md/dm-queue-length.c                    |   12 +++++--
 2 files changed, 48 insertions(+), 3 deletions(-)

Index: 2.6.30-rc5/Documentation/device-mapper/dm-queue-length.txt
===================================================================
--- /dev/null
+++ 2.6.30-rc5/Documentation/device-mapper/dm-queue-length.txt
@@ -0,0 +1,39 @@
+dm-queue-length
+===============
+
+dm-queue-length is a path selector module for device-mapper targets,
+which selects a path having the least number of in-flight I/Os.
+The path selector name is 'queue-length'.
+
+Table parameters for each path: [<repeat_count>]
+	<repeat_count>: The number of I/Os to dispatch using the selected
+			path before switching to the next path.
+			If not given, internal default is used. To check
+			the default value, see the activated table.
+
+Status for each path: <status> <fail-count> <in-flight>
+	<status>: 'A' if the path is active, 'F' if the path is failed.
+	<fail-count>: The number of path failures.
+	<in-flight>: The number of in-flight I/Os on the path.
+
+
+Algorithm
+=========
+
+dm-queue-length increments/decrements 'in-flight' when an I/O is
+dispatched/completed respectively.
+dm-queue-length selects a path having minimum 'in-flight'.
+
+
+Examples
+========
+In case that 2 paths (sda and sdb) are used with repeat_count == 128.
+
+# echo "0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128" \
+  dmsetup create test
+#
+# dmsetup table
+test: 0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128
+#
+# dmsetup status
+test: 0 10 multipath 2 0 0 0 1 1 E 0 2 1 8:0 A 0 0 8:16 A 0 0
Index: 2.6.30-rc5/drivers/md/dm-queue-length.c
===================================================================
--- 2.6.30-rc5.orig/drivers/md/dm-queue-length.c
+++ 2.6.30-rc5/drivers/md/dm-queue-length.c
@@ -35,7 +35,7 @@ struct path_info {
 	struct list_head	list;
 	struct dm_path		*path;
 	unsigned		repeat_count;
-	atomic_t		qlen;
+	atomic_t		qlen;	/* the number of in-flight I/Os */
 };
 
 static struct selector *alloc_selector(void)
@@ -113,13 +113,16 @@ static int ql_add_path(struct path_selec
 	struct path_info *pi;
 	unsigned repeat_count = QL_MIN_IO;
 
-	/* Parse the arguments */
+	/*
+	 * Arguments: [<repeat_count>]
+	 * 	<repeat_count>: The number of I/Os before switching path.
+	 * 			If not given, default (QL_MIN_IO) is used.
+	 */
 	if (argc > 1) {
 		*error = "queue-length ps: incorrect number of arguments";
 		return -EINVAL;
 	}
 
-	/* First path argument is number of I/Os before switching path. */
 	if ((argc == 1) && (sscanf(argv[0], "%u", &repeat_count) != 1)) {
 		*error = "queue-length ps: invalid repeat count";
 		return -EINVAL;
@@ -161,6 +164,9 @@ static int ql_reinstate_path(struct path
 	return 0;
 }
 
+/*
+ * Select a path having the minimum number of in-flight I/Os
+ */
 static struct dm_path *ql_select_path(struct path_selector *ps,
 				      unsigned *repeat_count, size_t nr_bytes)
 {