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Force mounting a VMFS datastore residing on a snapshot LUN results in the error: Cannot change the host configuration

Symptoms

As per the vSphere 4.0 SAN Configuration Guide, a VMFS datastore residing on a LUN that has been detected as a snapshot can be mounted using one of the following options:

Resignature the VMFS datastore and mount it on the selected host. Subsequent VMFS refresh is required on other hosts to see this newly resignatured datastore.
Keep the existing signature of the VMFS datastore and mount it on the selected host. This is also known as force mounting a VMFS datastore.

When force mounting a VMFS datastore, you may experience these symptoms:

Other hosts in the same datacenter cannot mount that same VMFS datastore from the vCenter Client.
The Resolve VMFS Volumes automatic task, seen in the task list of the vCenter Client, generates the error:

Error: Cannot change the host configuration.
Error Stack
Call "HostStorageSystem.ResolveMultipleUnresolvedVmfsVolumes" for object "storageVolume" on vCenter Server "MyVC" failed.

Where storageVolume is the name of the datastore and MyVC is the name of your vCenter Server.

Resolution

Cause

This issue may occur if:

Multiple ESX/ESXi 4.x and 5.0 hosts are managed by the same vCenter Server and these hosts are in the same datacenter.
A snapshot LUN containing a VMFS datastore is presented to all these ESX/ESXi hosts.
One of these ESX/ESXi hosts has force mounted the VMFS datastore that resides on this snapshot LUN.
A second ESX/ESXi host is attempting to do an operation at the same time.

When one ESX/ESXi host force mounts a VMFS datastore residing on a LUN which has been detected as a snapshot, an object is added to the datacenter grouping in the vCenter database to represent that datastore.

When the second ESX/ESXi host attempts to do the same operation on the same VMFS datastore, the operation fails because an object already exists within the same datacenter grouping in the vCenter database.

Since an object already exists, vCenter Server does not allow mounting the datastore on any other ESX host residing in that same datacenter.

Workaround

To mount the VMFS volume on each of the other ESX/ESXi hosts, use one of these options:

By connecting to the ESX host with vSphere Client:
1. Connect vSphere Client directly to the second ESX host as root.
2. Click the Configuration tab.
3. Click Storage.
4. Click Add Storage....
5. Complete the wizard to force mount the appropriate VMFS volume which is being detected as a Snapshot LUN.
By connecting directly to the ESX host service console:
1. Log in as root to the ESX host which cannot mount the datastore using an SSH client. For more information, see Unable to connect to an ESX host using Secure Shell (SSH) (1003807).
  Note: All of the commands listed are available in ESXi via the vSphere CLI.
2. Run the command:
  
  # esxcfg-volume -l
  
  The results appear similar to:
  
  VMFS3 UUID/label: 4b057ec3-6bd10428-b37c-005056ab552a/ TestDS
  Can mount: Yes
  Can resignature: Yes
  Extent name: naa.6000eb391530aa26000000000000130c:1 range: 0 - 1791 (MB)
  
  Record the UUID portion of the output. In the above example the UUID is 4b057ec3-6bd10428-b37c-005056ab552a.
  
  Note: The Can mount value must be Yes to proceed with this workaround.
  Note: The esxcfg-volume command has been depreciated in ESXi 5.0 in favor of the esxcli command. For more/related information, see vSphere handling of LUNs detected as snapshot (1011387).
3. Run the command:
  
  # esxcfg-volume -M <UUID>
  
  Where the <UUID> is the value recorded in step 3.
  
  Note: If you do not wish the volume mount to persist a reboot, the -m switch can be used instead.
By relocating the ESX hosts within the vCenter.

Alternatively, you can move each ESX host to its own datacenter before starting the Add Storage operation.

Warning: This option may be disruptive in a production environment, especially if there are more than two hosts in the same datacenter.

Note: To view the datastores again in vCenter Server, you may have to perform a rescan of the storage adapters on all ESX/ESXi hosts that the datastore is presented to or a refresh of the storage view.

Additional Information

There are two important notes to keep in mind when force mounting VMFS volumes on ESX/ESXi 4.x and 5.0 hosts:

A VMFS datastore can be mounted only if it does not collide with an already mounted VMFS datastore that has the same UUID (that is, VMFS signature). In other words, if the original LUN that contains the original VMFS datastore together with its snapshot LUN are both presented to the ESX server simultaneously, force mounting the VMFS datastore from the snapshot LUN on this same ESX host is not allowed. In this case, the only way to mount that VMFS datastore from the snapshot LUN is to resignature and choose a new label for that datastore.
In a datacenter that has multiple ESX/ESXi hosts, a VMFS datastore on a snapshot LUN can be force mounted on anyone of these hosts. When this datastore is mounted on one given host, that datastore cannot be force mounted on any other host using the vCenter user interface. To mount that datastore on other hosts, use either workaround A or B as described above.

For more information about Snapshot LUN detection, see Snapshot LUN detection in ESX 3.x and ESX 4 (1011385).

Update History

09/06/2011 - Added a note on rescanning storage adapters

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