There is at least one type of storage available on every virtual machine, which is the file system defined by the virtual machine image. This is the primary disk of the virtual machine, also called root disk. Nevertheless, the space available is typically limited and the lifetime of the data in it is limited by the virtual machine, i.e., the data is erased once the virtual machine is deleted. This article introduces the storage options available in CSC's cloud environment. Using the storage requires an account, a project, and a virtual machine.
Depending on the chosen flavor, a virtual machine may have a certain amount of additional local disk space, called ephemeral storage in the OpenStack terminology. This storage is visible on the virtual machine as a second hard disk.
The main advantage of the ephemeral storage is performance, as the data is stored in the hard disk of the server where the virtual machine is running. Nevertheless, the ephemeral storage is available only when the virtual machine is running. In addition, similarly to the data in the primary disk, the data in the ephemeral storage is lost too when you delete the virtual machine. Given that the data in the ephemeral storage is not replicated, you should not store any persistent data there, and the ephemeral storage should not be considered reliable.
You can find additional information on the ephemeral storage page.
If you are looking for a storage space which can be accessed like an additional hard disk, and which is not tied to a single virtual machine, you are looking for persistent volumes. You can attach a persistent volume to a virtual machine, and use it as you would use the ephemeral storage. Nevertheless, you can also detach it from the virtual machine and attach it to another virtual machine. In other words, you are free of deleting and recreating your virtual machines using the persistent volume, as a persistent volume is not tied to the lifetime of a single virtual machine. Moreover, persistent volumes represent a reliable storage option as the data in them is replicated.
You can find additional information on the persistent volumes page.
In case you need a storage space where your data can be easily accessed over the Internet, e.g., using URLs, object storage is the storage solution you are looking for. By using object storage, you are free to create and delete your virtual machines as the object storage is not tied to any virtual machine. Moreover, your data is replicated, thus object storage represent a reliable storage option.
At CSC, we offer Allas as our object storage solution. You can find additional information on the Allas page.
Sometimes you just want to have a backup copy of the data in a virtual machine or in a persistent volume. Alternatively, you might be interested to save the current state of a virtual machine, so that you can start other virtual machines cloning the same state. In these cases, you can use snapshots.
You can find additional information on the snapshots page.