Add guide for low-cost clusters (#1514)

Author: deliahu

Diff for: docs/guides/low-cost-clusters.md

@@ -0,0 +1,15 @@
+# Low-cost clusters
+
+_WARNING: you are on the master branch, please refer to the docs on the branch that matches your `cortex version`_
+
+Here are some tips for keeping costs down when running small clusters:
+
+* Consider using [spot instances](../cluster-management/spot-instances.md).
+
+* CPUs are cheaper than GPUs, so if there is low request volume and low latency is not critical, running on CPU instances will be more cost effective.
+
+* If traffic is low and you have multiple models, you may be able to save cost by serving all of your models from a single API, rather than using a separate API per model. This can be especially useful for GPU-based models. See our guide for [multi-model endpoints](multi-model.md).
+
+* If you need to have your cluster scale down to 0 API instances (the Cortex operator instance cannot be terminated), you must have `min_instances` set to 0 for your cluster, and no APIs can be running. Use `cortex get` to list your APIs, and `cortex delete <api_name>` to delete each one. After ~10 minutes, your cluster should scale down to 0 API instances.
+
+* By default, Cortex performs rolling updates on APIs, which means that during an update, additional instances may be required. If downtime during an update is acceptable, you can disable rolling updates. See [here](../troubleshooting/stuck-updating.md#disabling-rolling-updates) for instructions.

Diff for: docs/summary.md

@@ -68,6 +68,7 @@
 * [Multi-model endpoints](guides/multi-model.md)
 * [View API metrics](guides/metrics.md)
 * [Running in production](guides/production.md)
+* [Low-cost clusters](guides/low-cost-clusters.md)
 * [Set up a custom domain](guides/custom-domain.md)
 * [Set up VPC peering](guides/vpc-peering.md)
 * [SSH into worker instance](guides/ssh-instance.md)