When a storage device contains a VMFS datastore copy, you can mount the datastore with the existing signature or assign a new signature.
Each VMFS datastore created in a storage disk has a unique signature, also called UUID, that is stored in the file system superblock. When the storage disk is replicated or its snapshot is taken on the storage side, the resulting disk copy is identical, byte-for-byte, with the original disk. For example, if the original storage device contains a VMFS datastore with UUIDX, the disk copy appears to contain a datastore copy with the same UUIDX.
In addition to LUN snapshots and replications, certain device operations, such as LUN ID changes, might produce a copy of the original datastore.
ESXi can detect the VMFS datastore copy. You can mount the datastore copy with its original UUID or change the UUID. The process of changing the UUID is called the datastore resignaturing.
Whether you select resignaturing or mounting without resignaturing depends on how the LUNs are masked in the storage environment. If your hosts can see both copies of the LUN, then resignaturing is the optimal method.
Keeping Existing Datastore Signature
If you do not need to resignature a VMFS datastore copy, you can mount it without changing its signature.
You can keep the signature if, for example, you maintain synchronized copies of virtual machines at a secondary site as part of a disaster recovery plan. In the event of a disaster at the primary site, you mount the datastore copy and power on the virtual machines at the secondary site.
Resignaturing a VMFS Datastore Copy
Use datastore resignaturing if you want to retain the data stored on the VMFS datastore copy.
When resignaturing a VMFS copy, ESXi assigns a new signature (UUID) to the copy, and mounts the copy as a datastore distinct from the original. All references to the original signature in virtual machine configuration files are updated.
When you perform datastore resignaturing, consider the following points:
Datastore resignaturing is irreversible.
After resignaturing, the storage device replica that contained the VMFS copy is no longer treated as a replica.
A spanned datastore can be resignatured only if all its extents are online.
The resignaturing process is fault tolerant. If the process is interrupted, you can resume it later.
You can mount the new VMFS datastore without a risk of its UUID conflicting with UUIDs of any other datastore from the hierarchy of device snapshots.