[dm-devel] MPATH_MIN_IO

[Joe Thornber]

I've added a comment to explain the reasoning behind MPATH_MIN_IO.  It
wouldn't surprise me if we needed to reduce it (say to 100).

- Joe


--- diff/drivers/md/dm-mpath.c	2004-02-16 16:01:17.000000000 +0000
+++ source/drivers/md/dm-mpath.c	2004-02-16 15:59:44.000000000 +0000
@@ -18,8 +18,21 @@
 #include <linux/workqueue.h>
 #include <asm/atomic.h>
 
-/* FIXME: get rid of these */
+/* FIXME: get rid of this */
 #define MPATH_FAIL_COUNT	1
+
+/*
+ * We don't want to call the path selector for every single io
+ * that comes through, so instead we only consider changing paths
+ * every MPATH_MIN_IO ios.  This number should be selected to be
+ * big enough that we can reduce the overhead of the path
+ * selector, but also small enough that we don't take the policy
+ * decision away from the path selector.
+ *
+ * So people should _not_ be tuning this number to try and get
+ * the most performance from some particular type of hardware.
+ * All the smarts should be going into the path selector.
+ */
 #define MPATH_MIN_IO		1000
 
 /* Path properties */