software/kvm: Explain how to update boot-image-url-select

software/kvm/README.rst

@@ -117,3 +117,18 @@ files for more instance parameters (cpu-count, ram-size, disk-size, specific loc
 
 Then, if you want one of the two clones to takeover, you need to login into
 the hosting machine, go to the partition of the clone, and invoke bin/takeover.
+
+Technical notes
+---------------
+
+Updating boot-image-url-select
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ * download the new OS installation image
+ * calculate it's sha512sum and store as <SHA512>
+ * calculate it's md5sum and store as <MD5>
+ * upload it to shacache
+ * construct download url: ``https://shacache.nxdcdn.com/<SHA512>#<MD5>``
+ * update the ``boot-image-url-select`` in:
+    * ``instance-kvm-input-schema.json``
+    * ``instance-kvm-cluster-input-schema.json``