Managing Duplicate VMFS Datastores - Resignaturing

http://www.vmware.com/pdf/vsphere4/r41/vsp_41_esx_server_config.pdf page 119

When a LUN 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 LUN has a unique UUID that is stored in the file system superblock. When the LUN is replicated or snapshotted, the resulting LUN copy is identical, byte-for-byte, with the original LUN. As a result, if the original LUN contains a VMFS datastore with UUID X, the LUN copy appears to contain an identical VMFS datastore, or a VMFS datastore copy, with exactly the same UUID X. ESX can determine whether a LUN contains the VMFS datastore copy, and either mount the datastore copy with its original UUID or change the UUID, thus resignaturing the datastore.

Mounting VMFS Datastores with Existing Signatures
You might not have to resignature a VMFS datastore copy. You can mount a VMFS datastore copy without changing its signature. For example, you can 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 can mount the datastore copy and power on the virtual machines at the secondary site.

IMPORTANT You can mount a VMFS datastore copy only if it does not collide with the original VMFS datastore that has the same UUID. To mount the copy, the original VMFS datastore has to be offline.

When you mount the VMFS datastore, ESX allows both reads and writes to the datastore residing on the LUN copy. The LUN copy must be writable. The datastore mounts are persistent and valid across system reboots. Because ESX does not allow you to resignature the mounted datastore, unmount the datastore before resignaturing.

Mount a VMFS Datastore with an Existing Signature
If you do not need to resignature a VMFS datastore copy, you can mount it without changing its signature.

Prerequisites
Before you mount a VMFS datastore, perform a storage rescan on your host so that it updates its view of LUNs presented to it.

Procedure

1. Log in to the vSphere Client and select the server from the inventory panel.
2. Click the Configuration tab and click Storage in the Hardware panel.
3. Click Add Storage.
4. Select the Disk/LUN storage type and click Next.
5. From the list of LUNs, select the LUN that has a datastore name displayed in the VMFS Label column and click Next. The name present in the VMFS Label column indicates that the LUN is a copy that contains a copy of an existing VMFS datastore.
6. Under Mount Options, select Keep Existing Signature.
7. In the Ready to Complete page, review the datastore configuration information and click Finish.

What to do next
If you later want to resignature the mounted datastore, you must unmount it first.

Resignaturing VMFS Copies
Use datastore resignaturing to retain the data stored on the VMFS datastore copy. When resignaturing a VMFS copy, ESX assigns a new UUID and a new label to the copy, and mounts the copy as a datastore distinct from the original. The default format of the new label assigned to the datastore is snap-snapID-oldLabel, where snapID is an integer and oldLabel is the label of the original datastore. When you perform datastore resignaturing, consider the following points:

• Datastore resignaturing is irreversible.
• The LUN copy that contains the VMFS datastore that you resignature is no longer treated as a LUN copy.
• A spanned datastore can be resignatured only if all its extents are online.
• The resignaturing process is crash and 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 colliding with UUIDs of any other datastore, such as an ancestor or child in a hierarchy of LUN snapshots.

Resignature a VMFS Datastore Copy
Use datastore resignaturing if you want to retain the data stored on the VMFS datastore copy.

Prerequisites
To resignature a mounted datastore copy, first unmount it. Before you resignature a VMFS datastore, perform a storage rescan on your host so that the host updates its view of LUNs presented to it and discovers any LUN copies.

Procedure

1. Log in to the vSphere Client and select the server from the inventory panel.
2. Click the Configuration tab and click Storage in the Hardware panel.
3. Click Add Storage.
4. Select the Disk/LUN storage type and click Next.
5. From the list of LUNs, select the LUN that has a datastore name displayed in the VMFS Label column and click Next. The name present in the VMFS Label column indicates that the LUN is a copy that contains a copy of an existing VMFS datastore.
6. Under Mount Options, select Assign a New Signature and click Next.
7. In the Ready to Complete page, review the datastore configuration information and click Finish.

What to do next
After resignaturing, you might have to do the following:

• If the resignatured datastore contains virtual machines, update references to the original VMFS datastore in the virtual machine files, including .vmx, .vmdk, .vmsd, and .vmsn.
• To power on virtual machines, register them with vCenter Server.