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, the following device operations cause ESXi to mark the datastore on the device as a copy of the original datastore:
LUN ID changes
SCSI device type changes, for example, from SCSI-2 to SCSI-3
SPC-2 compliancy enablement
ESXi can detect the VMFS datastore copy and display it in the vSphere Web Client. 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.