etcdBackendQuotaLowSpace #

Meaning #

This alert fires when the total existing DB size exceeds 95% of the maximum DB quota. The consumed space is in Prometheus represented by the metric etcd_mvcc_db_total_size_in_bytes, and the DB quota size is defined by etcd_server_quota_backend_bytes.

Impact #

In case the DB size exceeds the DB quota, no writes can be performed anymore on the etcd cluster. This further prevents any updates in the cluster, such as the creation of pods.

Diagnosis #

The following two approaches can be used for the diagnosis.

CLI Checks #

To run etcdctl commands, we need to rsh into the etcdctl container of any etcd pod.

$ NAMESPACE="kube-etcd"
$ kubectl rsh -c etcdctl -n $NAMESPACE $(kubectl get po -l app=etcd -oname -n $NAMESPACE | awk -F"/" 'NR==1{ print $2 }')

Validate that the etcdctl command is available:

$ etcdctl version

etcdctl can be used to fetch the DB size of the etcd endpoints.

$ etcdctl endpoint status -w table

PromQL queries #

Check the percentage consumption of etcd DB with the following query in the metrics console:

(etcd_mvcc_db_total_size_in_bytes / etcd_server_quota_backend_bytes) * 100

Check the DB size in MB that can be reduced after defragmentation:

(etcd_mvcc_db_total_size_in_bytes - etcd_mvcc_db_total_size_in_use_in_bytes)/1024/1024

Mitigation #

Capacity planning #

If the etcd_mvcc_db_total_size_in_bytes shows that you are growing close to the etcd_server_quota_backend_bytes, etcd almost reached max capacity and it’s start planning for new cluster.

In the meantime before migration happens, you can use defrag to gain some time.

Defrag #

When the etcd DB size increases, we can defragment existing etcd DB to optimize DB consumption as described in etcdDefragmentation. Run the following command in all etcd pods.

$ etcdctl defrag

As validation, check the endpoint status of etcd members to know the reduced size of etcd DB. Use for this purpose the same diagnostic approaches as listed above. More space should be available now.