add a caveat

tsa/howto/upgrades/buster.mdwn

 [[!toc]]
 
+Caveats
+=======
+
+ * Warning: upgrading(restarting) openvswitch will mean all guests lose network.
+
+ * At least on kvm5, I had to reorder inet6 and inet4 stanzas in /etc/network/interfaces
+   for brpub to have an ipv6 address and route to the guest network after the
+   upgrade.
+
+ * On physical hosts witch /etc/sysfs.d/local-io-schedulers.conf, note that 'deadline' no
+   longer existsts.  Probably it is also not necessary as Linux might pick the right
+   scheduler anyhow.
+
 Procedure
 =========