[PATCH] md: Allow the write_mostly flag to be set via sysfs

It appears in /sys/mdX/md/dev-YYY/state
and can be set or cleared by writing 'writemostly' or '-writemostly'
respectively.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/md.txt

@@ -309,6 +309,9 @@ Each directory contains:
 	      faulty   - device has been kicked from active use due to
                          a detected fault
 	      in_sync  - device is a fully in-sync member of the array
+	      writemostly - device will only be subject to read
+		         requests if there are no other options.
+			 This applies only to raid1 arrays.
 	      spare    - device is working, but not a full member.
 			 This includes spares that are in the process
 			 of being recoverred to
@@ -316,6 +319,8 @@ Each directory contains:
 	This can be written to.
 	Writing "faulty"  simulates a failure on the device.
 	Writing "remove" removes the device from the array.
+	Writing "writemostly" sets the writemostly flag.
+	Writing "-writemostly" clears the writemostly flag.
 
       errors
 	An approximate count of read errors that have been detected on

drivers/md/md.c

@@ -1737,6 +1737,10 @@ state_show(mdk_rdev_t *rdev, char *page)
 		len += sprintf(page+len, "%sin_sync",sep);
 		sep = ",";
 	}
+	if (test_bit(WriteMostly, &rdev->flags)) {
+		len += sprintf(page+len, "%swrite_mostly",sep);
+		sep = ",";
+	}
 	if (!test_bit(Faulty, &rdev->flags) &&
 	    !test_bit(In_sync, &rdev->flags)) {
 		len += sprintf(page+len, "%sspare", sep);
@@ -1751,6 +1755,8 @@ state_store(mdk_rdev_t *rdev, const char *buf, size_t len)
 	/* can write
 	 *  faulty  - simulates and error
 	 *  remove  - disconnects the device
+	 *  writemostly - sets write_mostly
+	 *  -writemostly - clears write_mostly
 	 */
 	int err = -EINVAL;
 	if (cmd_match(buf, "faulty") && rdev->mddev->pers) {
@@ -1766,6 +1772,12 @@ state_store(mdk_rdev_t *rdev, const char *buf, size_t len)
 			md_new_event(mddev);
 			err = 0;
 		}
+	} else if (cmd_match(buf, "writemostly")) {
+		set_bit(WriteMostly, &rdev->flags);
+		err = 0;
+	} else if (cmd_match(buf, "-writemostly")) {
+		clear_bit(WriteMostly, &rdev->flags);
+		err = 0;
 	}
 	return err ? err : len;
 }