Ephemeral Storage
This document describes one of the two fundamental key virtual machine storage concepts in Private Cloud Director - Ephemeral Storage.
What is Ephemeral Storage
Ephemeral Storage in Private Cloud Director refers to temporary, non-persistent storage associated with a virtual machine instance. It's like a local hard drive for the virtual machine, and its data is lost when the VM is deleted. This contrasts with persistent / block storage (like volumes) which remains intact even after the VM is deleted.
Used For
Ephemeral storage in Private Cloud Director is available as an option for a virtual machine's root disk only. Any other non root disk partitions required for a VM must be mounted as block storage volumes.
How to Choose
You choose ephemeral storage for the root disk of a VM at VM creation or rebuild time, by choosing one of the following options as part of VM creation in Private Cloud Director UI:
- Boot from Image
- Boot from VM Snapshot
Read more here about VM Ephemeral Disk Snapshot.
When you choose this option:
- The virtual machine root disk is created on ephemeral storage, using the image or the VM Ephemeral Disk Snapshot that you choose.
- The ephemeral disk is used to run the virtual machine guest operating system and boot partition.
- Under the hood, the image file is copied over to the ephemeral storage location on the hypervisor host where it's cached for future use. A VM is then created usually by spawning a delta disk from the image's base disk.
Lifecycle
Ephemeral storage is tied to the lifecycle of a VM. When the VM is created, the ephemeral disk is created, and when the VM is deleted, the ephemeral disk is also deleted. Virtual machines using ephemeral storage as boot option are created with one ephemeral disk as the VM's root disk, which is then used to run the VM's guest operating system and boot partition. It's important to remember that even if the hypervisor host may be using NFS shared storage to store the VM's ephemeral disks, the disk will still be deleted when the VM gets deleted.
Use Case
Ephemeral storage is useful for test environments or for cloud-native applications where the VM's root disk need not be persisted post VM deletion.
Configure Ephemeral Storage
- An Administrator configures ephemeral storage location for all hypervisors as part of Cluster Blueprint by specifying the Virtual machine storage path while configuring the blueprint.
- This path may map to local storage or NFS shared storage, depending on how that specific storage path is mounted or configured under the hood at per hypervisor level.
Ephemeral Storage Types
Ephemeral storage can be further classified into two sub-types:
Ephemeral Local Storage
You are using ephemeral local storage when the Virtual machine storage path that you specify as part of your cluster blueprint maps to local storage on your hypervisor hosts. The storage can be only be accessed by the host and the VMs provisioned on it. If the host crashes, the VMs running on it can not be recovered without recovering the host.
Ephemeral Shared Storage
You are using ephemeral shared storage when the Virtual machine storage path that you specify as part of your cluster blueprint maps to an NFS mount or other type of shared storage. This type of ephemeral storage has following advantages:
- Fast live migration - VM live migration will be faster and will put less load on the network when using shared ephemeral storage as the VM root disk does not need to be copied from source host to destination host.
- VM recovery - This also allows for easy recovery of VMs when a hypervisor crashes or dies, as long as the NFS mount is accessible to at least one other hypervisor node.