UVM enables storage that is accessed by one cluster to be taken over by another cluster. Using UVM instead of Universal Migrator enables the NAS server to preserve snapshots, quotas and ACLs. It also has the ability to replicate a whole span in a single operation.
The server detects UVM separately for each span. On a single system, some spans can reside on plain storage, some on HDP and some on UVM internal LUs.
UVM cannot be used with spans or file systems that are not supported on the new cluster. These include:
- Spans with only one admin area per system drive.
- File systems with object-based snapshots.
In these circumstances, use Universal Migrator instead of UVM.
UVM works with external and internal LUs. External LUs are LUs presented by other storage systems; the local storage system virtualizes these external LUs and presents them to the server as internal LUs, which appear to the server as if they resided on the storage system's own disks.
If the LUs to be virtualized are relatively large, the administrator must create a pool on the local storage with data direct mapping (DDM) enabled. The administrator uses the external LUs as the underlying storage for this DDM pool, and each DP-Vol in the DDM pool, which is presented to the server as an internal LU, corresponds exactly to one of these external LUs.
If the external LUs are relatively small, the administrator can still create a DDM pool, but may instead opt to virtualize the LUs directly.
In either case, the server can detect UVM internal LUs, and automatically treats the span as residing on UVM storage.
When using UVM, it is preferable to create the HDP pool on the system containing the external logical devices, therefore the HNAS will treat the HDP pool as if it were local disk in an HDP pool and not a pool of UVM disks.
Identifying a UVM span using the CLI
In the span-list command, for a UVM span, the 'Con' column displays the letters 'UVM'. However, the span-list -s command does not show how much free space is available at storage level, because that information is not available to the server.