Add doc to manage PVC in Elasticsearch FAQ

There is a lot of questions about the procedure to resize Elasticsearch PVC by migrating to new chart release (https://github.com/elastic/helm-charts/issues/647#issuecomment-636778450). This should be documented in the FAQ.

In the same way, we should document the way to create PVC matching VolumeClaimTemplates naming convention outside of Elasticsearch chart (https://github.com/elastic/helm-charts/issues/518#issuecomment-726109728).

This should fix #772, #851, #518 & #656

[czomo]

I managed to resize volumes in kubernetes using ALLOWVOLUMEEXPANSION available in later versions

kubectl get Storageclass
NAME            PROVISIONER             RECLAIMPOLICY   VOLUMEBINDINGMODE      ALLOWVOLUMEEXPANSION   AGE
gp2 (default)   kubernetes.io/aws-ebs   Delete          WaitForFirstConsumer   true                   93d

if ALLOWVOLUMEEXPANSION is not set to true add allowVolumeExpansion: true kubectl edit storageclass and add

kind: StorageClass
apiVersion: storage.k8s.io/v1
provisioner: kubernetes.io/aws-ebs
allowVolumeExpansion: true

Then go to PVC and change volume size. WARNING: Do not change size 2 in one row, i managed to get errors.

AWS modifyVolume failed for vol-adasdasdasdasd with IncorrectModificationState: Volume vol-adasdasdasdasd cannot be modified in modification state OPTIMIZING

EDIT I found out that before resizing we need to find out the state of volume, completed is our desired state aws ec2 describe-volumes-modifications --volume-ids vol-xxxxxxxxxxxxxxxxxx --region us-west-2

[MaxymVlasov]

Hey, @jmlrt, I see that you work in Elastic as SRE. Maybe you can be able to ping folks in internal slack etc. to escalate that issue? I know that docs writing can be more difficult than implementing feature/bug fixes, but 1 year for docs issue… it time to escalation, I think 🙂