The storage that is included on each node in the cluster is ephemeral fast SSD.
By default we do not guarantee the safety of any new data you create on your cluster. The underlying VMs running your code could be replaced at any moment for maintenance. When that happens your containers are safely migrated to new servers, but without any data created in any mounted volumes.
Consider the local storage as a temporary cache to use between deployments.
Do not store there any important data as it will be lost.
Permanent storage is an additional service that can be purchased separately and it's charged per GB.