[dm-devel] [PATCH v2 3/3] multipathd: avoid unnecessary path read-only reloads

[Benjamin Marzinski]

A mulitpath device can only be reloaded read/write when all paths are
read/write. Also, whenever a read-only scsi device is rescanned, the
scsi subsystem will first unconditionally issue a uevent with DISK_RO=0
before checking the read-only status, and if it the device is still
read-only, issuing another uevent with DISK_RO=1. These uevents cause
pointless reloads when read-only paths are rescanned. To avoid this,
first check if the path is being changed to the existing multipath
read-only state. If the state is the same, do nothing. If it's
different, check to see if all paths are read/write before changing a
multipath device from read-only to read/write. If the multipath device
read-only state is unknown, assume that it needs to be reloaded.

Signed-off-by: Benjamin Marzinski <bmarzins at redhat.com>
---
 multipathd/main.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 47 insertions(+), 1 deletion(-)

diff --git a/multipathd/main.c b/multipathd/main.c
index 2e99a266..009a99d9 100644
--- a/multipathd/main.c
+++ b/multipathd/main.c
@@ -1440,6 +1440,52 @@ finish_path_init(struct path *pp, struct vectors * vecs)
 	return -1;
 }
 
+static int
+sysfs_get_ro (struct path *pp)
+{
+	int ro;
+	char buff[3]; /* Either "0\n\0" or "1\n\0" */
+
+	if (!pp->udev)
+		return -1;
+
+	if (sysfs_attr_get_value(pp->udev, "ro", buff, sizeof(buff)) <= 0) {
+		condlog(3, "%s: Cannot read ro attribute in sysfs", pp->dev);
+		return -1;
+	}
+
+	if (sscanf(buff, "%d\n", &ro) != 1 || ro < 0 || ro > 1) {
+		condlog(3, "%s: Cannot parse ro attribute", pp->dev);
+		return -1;
+	}
+
+	return ro;
+}
+
+static bool
+needs_ro_update(struct multipath *mpp, int ro)
+{
+	struct pathgroup * pgp;
+	struct path * pp;
+	unsigned int i, j;
+
+	if (!mpp || ro < 0)
+		return false;
+	if (!has_dm_info(mpp))
+		return true;
+	if (mpp->dmi.read_only == ro)
+		return false;
+	if (ro == 1)
+		return true;
+	vector_foreach_slot (mpp->pg, pgp, i) {
+		vector_foreach_slot (pgp->paths, pp, j) {
+			if (sysfs_get_ro(pp) == 1)
+				return false;
+		}
+	}
+	return true;
+}
+
 static int
 uev_update_path (struct uevent *uev, struct vectors * vecs)
 {
@@ -1512,7 +1558,7 @@ uev_update_path (struct uevent *uev, struct vectors * vecs)
 		}
 
 		ro = uevent_get_disk_ro(uev);
-		if (mpp && ro >= 0) {
+		if (needs_ro_update(mpp, ro)) {
 			condlog(2, "%s: update path write_protect to '%d' (uevent)", uev->kernel, ro);
 
 			if (mpp->wait_for_udev)
-- 
2.17.2