diff --git a/contrib/ci/lambda_functions/ci.py b/contrib/ci/lambda_functions/ci.py
--- a/contrib/ci/lambda_functions/ci.py
+++ b/contrib/ci/lambda_functions/ci.py
@@ -572,6 +572,13 @@
     # there are no available spot instances. But we handle this by setting a
     # short request validity window and retrying in a different availability
     # zone. Eventually we should find someone willing to satisfy our request.
+    #
+    # TODO remove BlockDurationMinutes for even greater savings. By requesting
+    # a full block, we essentially create a 1 hour reservation rather than an
+    # instance that can be terminated whenever. The stronger guarantees make
+    # it more expensive. If we remove this, we will no longer have
+    # ActualBlockHourlyPrice in the spot request metdata and will need to
+    # update hourly cost accounting elsewhere in this file.
     res = ec2.request_spot_instances(
         BlockDurationMinutes=60,
         ValidUntil=datetime.datetime.utcnow() + datetime.timedelta(minutes=1),