Decide between dns and dnssrvnoa when connecting to memcached

[friedrichg]

Also @friedrichg you guys over at jsonnet use dnssrvnoa I believe for DNS service discovery, while we here use dns. Should we change that?

Originally posted by @nschad in #551 (comment)

[friedrichg]

Services in cortex-jsonnet expose more than 1 port.

apiVersion: v1
kind: Service
metadata:
  labels:
    name: memcached-index-queries
  name: memcached-index-queries
  namespace: default
spec:
  clusterIP: None
  ports:
  - name: memcached-client
    port: 11211
    targetPort: 11211
  - name: exporter-http-metrics
    port: 9150
    targetPort: 9150
  selector:
    name: memcached-index-queries

But flag uses the port in the url.

https://github.com/cortexproject/cortex-jsonnet/blob/c99aab2b2aca15b54648875a60a20869e91eb018/cortex/tsdb-config.libsonnet#L41

cortex-helm-chart/templates/_helpers.tpl

Line 90 in 706f190

- "-blocks-storage.bucket-store.index-cache.memcached.addresses=dns+{{ .Release.Name }}-memcached-blocks-index.{{ .Release.Namespace }}.svc.{{ .Values.clusterDomain }}:11211"

I think having a separate service for memcached metrics is more sound when having large number of memcacheds servers, we don't want to reach endpoint capacity in the service endpoints when using dns.

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I will do a couple of more tests before suggesting a change