Managing Disk Groups And Devices

When you enable Virtual SAN on a cluster, choose a disk-claiming mode to organize devices into groups.

Automatic Mode

In automatic mode, Virtual SAN automatically discovers and claims all local, empty, and usable devices on each host and organizes them into default disk groups with one cache and one or multiple capacity devices on each host in the cluster. If you add more capacity to the hosts or add new hosts with capacity to the Virtual SAN cluster, the local storage on the host is automatically claimed by Virtual SAN and the total capacity of the Virtual SAN datastore increases. Virtual SAN in automatic mode claims only local devices on the Virtual SAN hosts in the cluster. In an all-flash cluster, you must manually mark the flash devices to use for capacity. In an environment without any HDD disks, if there are no flash disks marked for capacity flash, Virtual SAN in automatic mode won't claim any disks or create any disk groups. See Mark Flash Devices as Capacity Using ESXCLI or Mark Flash Devices as Capacity using RVC.

Note: When hosts are using SAS controllers, Virtual SAN might identify certain devices as remote and unable to automatically claim devices as local, and those devices are displayed as remote. For those devices, you can manually create disk groups, even though the cluster is configured to claim disks automatically. You can also manually add any remote non-shared devices.

After Virtual SAN has automatically claimed devices, it creates the Virtual SAN shared datastore. The total size of the datastore reflects the capacity of all capacity devices in disk groups across all hosts in the cluster, except for some overhead used for metadata. On-disk format version 1.0 adds an additional overhead of approximately 1 GB per capacity device. On-disk format version 2.0 adds an additional overhead, typically no more than 1-2 percent capacity per device. On-disk format version 3.0 and later adds an additional overhead, typically no more than 1-2 percent capacity per device. Deduplication and compression with software checksum enabled require additional overhead of approximately 6.2 percent capacity per device.

Manual Mode

In manual mode, you must manually specify each host and each device to be used for the Virtual SAN datastore. You can organize devices into disk groups using the semi-automatic or manual method.

When you use the semi-automatic method, Virtual SAN claims and organizes the devices that you specify into default disk groups.

When you use the manual method, you define disk groups and individually select devices to include in each disk group. When you manually create a disk group, consider the ratio of flash cache to consumed capacity. Although the ratio depends on the requirements and workload of the cluster, consider using at least 10 percent of flash cache to consumed capacity ratio (not including replicas such as mirrors).

In manual mode, the Virtual SAN cluster initially contains a single Virtual SAN datastore with zero bytes consumed.

As you create disk groups on each host and add at least one cache and one or more capacity devices to each of the disk groups, the size of the datastore grows according to the amount of physical capacity added to the cluster. Virtual SAN creates a single distributed Virtual SAN datastore using the local empty capacity available from the hosts added to the cluster.

If the cluster requires multiple flash cache devices, you must create multiple disk groups manually, because a maximum of one flash cache device is allowed per disk group.

Note: If a new ESXi host is added to the Virtual SAN cluster, which has been set up in manual mode, the local storage from that host is not added to the Virtual SAN Datastore automatically. You have to manually create disk groups and add the devices to the disk group in order to use the new storage from the new ESXi host.

Manual mode provides a semi-automatic method and a manual method to claim devices. If you use the semi-automatic method, you can select multiple devices to be consumed by Virtual SAN, and Virtual SAN creates default disk groups for you. If you use the manual method, you can organize selected devices into disk groups. You first select a cache device and then add one or more capacity devices to form a disk group.