Nova Ephemeral Storage¶
This is the default OpenStack storage option used for creating VMs. Virtual machine instances are typically created with at least one ephemeral disk which is used to run the VM guest operating system and boot partition.
Ephemeral storage for VMs, which includes swap disk storage, ephemeral disk storage, and root disk storage if the VM is configured for boot-from-image, is implemented by the nova service. For flexibility and scalability, this storage is defined using a nova-local local volume group created on the compute hosts.
The nova-local group can be backed locally by one or more disks or partitions on the compute host, or remotely by resources on the internal Ceph cluster (on controller or storage hosts). If it is backed locally on the compute host, then it uses CoW-image storage backing. For more information about nova-local backing options, see StarlingX Storage Configuration and Management: Block Storage for Virtual Machines.
Compute hosts are grouped into host aggregates based on whether they offer CoW or remote Ceph-backed local storage. The host aggregates are used for instantiation scheduling.
For storage on compute hosts, CoW-image backing uses an instances logical volume, or Instances LV. This contains the /etc/nova/instances file system, and is used for the following:
the nova image cache, containing images downloaded from Glance
various small nova control and log files, such as the libvirt.xml file, which is used to pass parameters to libvirt at launch, and the console.log file
For CoW-image-backed local storage, Instances LV is also used to hold CoW-image disk files for use as VM disk storage. It is the only volume in nova-local.
By default, no size is specified for the Instances LV. For non-root disks, the minimum required space is 2 GB for a nova-local volume group with a total size less that 80 GB, and 5 GB for a nova-local volume group larger or equal than 80 GB; you must specify at least this amount. You can allocate more Instances LV space to support the anticipated number of boot-from-image VMs, up to 50% of the maximum available storage of the local volume group. At least 50% free space in the volume group is required to provide space for allocating logical volume disks for launched instances. The value provided for the Instance LV Size is limited by this maximum.
Instructions for allocating the Instances LV Size using the Web administration interface or the CLI are included in as part of configuring the compute nodes. Suggested sizes are indicated in the Web administration interface.
If less than the minimum required space is available, the compute host cannot be unlocked.