Ephemeral Storage

Ephemeral storage is the default storage option used for creating VMs in Private Cloud Director . Virtual machines using this storage option are created with one ephemeral disk which is used to run the VM guest operating system and boot partition.

An ephemeral disk is deleted when the virtual machine is deleted. Ephemeral disks are often used with cloud native applications where VMs are expected to be short-lived and the data does not need to persist beyond the life of a given VM.

An Administrator makes ephemeral storage available to Private Cloud Director by allocating either local storage on a hypervisor or NFS shared storage volume mounted on a hypervisor as the default storage location for virtual machines. This is typically done while onboarding a new hypervisor to Private Cloud Director.

Ephemeral Disks w/ Local Storage

Ephemeral disks are typically created using the local/direct-attached storage of the hypervisor node. The storage can be only be accessed by the compute node running on that host server and the VMs it hosts.

Ephemeral Disks w/ NFS

Another option for Ephemeral storage is to host them on external storage via an NFS mount. In this case, the data is still purged when the VM using that the virtual disk on the file share is deleted. However, using an NFS mount for the VM root disk does allow for simpler VM migration since multiple compute nodes can be given access to the same file share. This method also allows for recovery of VMs when a hypervisor crashes or dies, as long as the NFS mount is accessible to at least one other hypervisor node.

Figure 2: Ephemeral Disk with NFS

When you create a virtual machine, unless you explicitly choose a block storage volume to create your VM from, the default storage used is ephemeral storage. During VM creation, once Private Cloud Director placement engine finds the appropriate hypervisor to place the VM on, the image file is copied over to the local storage of the hypervisor where it's cached for future use. A VM is then created usually by spawning a delta disk from the image's base disk.

Comparison of Ephemeral vs Block Storage

Following table provides a comparison between Private Cloud Director ephemeral vs block storage.

		Ephemeral Storage	Block Storage
Used to		Run Operating System and scratch space	Store persistent data on devices accessed as hard drives
Accessed through		A file system	A block device
Accessible		With a VM	With a VM
Managed by		Private Cloud Director Compute Service	Private Cloud Director Block Storage Service
Persists until		VM is deleted. VM root disk is deleted when the VM gets deleted.	Volume is deleted. Even when the VM gets deleted, the volume persists until explicitly deleted.
Sizing determined by		Administrator configures size settings as part of VM Flavors. Limited by local storage available on hypervisors.	Specified by user as part of volume creation
Multiple disks in a VM		Adding multiple ephemeral disks to a VM is not supported.	You can have multiple volumes mounted on a VM.
Example of typical usage		10GB first disk, 30GB second disk	1 TB block storage volume
Advantages		- Simple - Out of box and easy to use for end-users - Cheap and easy to provision for Administrators - provided using local storage (or NFS shared storage) available to hypervisor.	- Allows for advance tiering of storage - by creating key- value pair metadata with the block storage endpoint. - Can dynamically resize block storage volumes (even after allocating to a VM) - Can add multiple volumes to a VM. - Can pass Quality-of-Service metadata to the storage controller to get required IOPs for the storage volume (assuming support by the Cinder plugin).
Disadvantages		- Does not support advance features like resizing the disk once created - Adding multiple ephemeral disks to a VM is not supported. - No ability to associate Quality of Service metadata with VM disk - No support for Storage Tiering.

Block Storage

Block storage in Private Cloud Director enables persistent block storage volumes, and is one of the most important and popular Private Cloud Director storage options. Private Cloud Director block storage service provides VMs with block storage volumes that persist even when the VMs they are attached to are deleted.

Info

A single block volume can only be attached to a single VM at a given time, but provides the flexibility to be detached from one VM and attached to another.

Block storage is compatible with VM live migration, in that data volumes get moved as needed when doing a VM migration.

Advantages of block storage:

• If for some reason you need to delete and relaunch a virtual machine, you can keep any non-disposable data on the block storage volumes and re-attached them to the new VM.

• If a hypervisor node crashes, you can launch VMs on surviving hypervisors and attach block storage volumes to those VMs with the data intact.

• You can resize and extend a block storage volume if you need to grow the capacity of a disk being used by a VM.

• Using a dedicated storage node or storage subsystem to host volumes, capacity can be provided that is greater than what is available via the direct-attached storage in the compute nodes (Note that is also true if using NFS with shared storage but without data persistence).

• You can leverage specific enterprise storage features such as thin-provisioning, tiering, Quality of Service, etc. when using Private Cloud Director block storage.

• A block storage volume can be used as the boot disk for a virtual machine; in that scenario, an ephemeral disk is not required.