Backend performance requirements for OpenShift etcd

Issue

• etcd performance can be impacted by poor storage and network performance, causing multiple errors:

  $ oc logs --follow=true etcd-ocp4-9wwcf-master-0 -c etcd -n openshift-etcd
  ...
  etcdserver: failed to send out heartbeat on time (exceeded the 100ms timeout for xxx ms)
  etcdserver: server is likely overloaded
  etcdserver: read-only range request "key:\"xxxx" count_only:true " with result "xxxx" took too long (xxx s) to execute
  etcdserver: read-only range request "key:\"xxxx" count_only:true " with result "xxxx" took too long (xxxx ms) to execute
  etcdserver: read-only range request "xxxx" with result "xxxx" took too long (xxx ms) to execute
  wal: sync duration of xxxx s, expected less than 1s