Storage array management tools allow us to clone/snapshot/replicate LUN/s backing VMFS datastore/s. The snapshots are created at the storage array using the array management interface. However, it is not possible to mount an identical copy of an existing VMFS datastore to the same ESXi host. This is deliberately disallowed to avoid data corruption, unless it is backed by a different device ID.
ESXi identifies each VMFS volume using its signature denoted by a UUID (Universally Unique Identifier). The UUID is generated when the volume is first created or resignatured and is stored in the LVM header of the VMFS volume.
The command esxcli storage filesystem list can be used to list all the VMFS datastores along with their details, which include the UUID, in columnar format as follows:
The following diagram shows the structure of a VMFS UUID:
When an ESXi host scans for new LUN devices and VMFS volumes on it, it compares the physical device ID (NAA ID) of the LUN with the device ID (NAA ID) value stored in the VMFS volumes LVM header. If it finds a mismatch, then it flags the volume as a snapshot volume. Volumes detected as snapshots are not mounted by default.
There are two options to mount such volumes/datastores. These options are made available via the New Datastore wizard, only if you select a clone/snapshot/replica LUN with a VMFS volume on it:
- The first option is to generate a new signature for the VMFS volume before mounting it. This has to be used if you are mounting a clone or a snapshot of an existing VMFS datastore to the same host/s. The option is presented as Assign a new signature in the New Datastore wizard. The process of assigning a new signature will not only update the LVM header with the newly generated UUID, but all the Physical Device ID (NAA ID) of the snapshot LUN. Here, the VMFS volume/datastore will be renamed by prefixing the word snap followed by a random number and the name of the original datastore:
- The second option is to mount the VMFS volume without generating a signature or updating the LVM header of the detected volume. The option is presented as Keep existing signature in the New Datastore wizard. This is used when you are attempting to temporarily mount the snapshot volume on an ESXi that doesn't see the original volume. If you were to attempt mounting the VMFS volume by keeping the existing signature and if the host sees the original volume, then you will not be allowed to mount the volume and will be warned about the presence of another VMFS volume with the same UUID:
However, as mentioned before, the volume can be mounted onto an ESXi host that does not see the original datastore, with the same datastore name.