Add some additional documentation on how the sysvm disk locking works

svn path=/trunk/packages/invirt-xen-config/; revision=1462

diff --git a/debian/invirt-xen-config.init b/debian/invirt-xen-config.init
index 02dedc1..ec23d0d 100755 (executable)
--- a/debian/invirt-xen-config.init
+++ b/debian/invirt-xen-config.init
@@ -39,7 +39,11 @@ start_sysvm() {
     fi
     
     if lvchange -a n "$DISK" >/dev/null 2>&1 && lvchange -a ey "$DISK" >/dev/null 2>&1; then
     fi
     
     if lvchange -a n "$DISK" >/dev/null 2>&1 && lvchange -a ey "$DISK" >/dev/null 2>&1; then

-        # If we can lock on the VM's disk, then the VM isn't running
+        # If we can disable and then re-enable the VMs disk, then the
+        # VM can't be running. If the lvchange -a ey succeeds, then we
+        # have an exclusive lock across the cluster on enabling the
+        # disk, which avoids the potential race condition of two hosts
+        # starting a VM at the same time

         [ "$VERBOSE" != no ] && log_daemon_msg "Starting sysvm $VM"
         xm create "sysvms/$VM" >/dev/null
         [ "$VERBOSE" != no ] && log_end_msg $?
         [ "$VERBOSE" != no ] && log_daemon_msg "Starting sysvm $VM"
         xm create "sysvms/$VM" >/dev/null
         [ "$VERBOSE" != no ] && log_end_msg $?