VMFS Re-Signaturing

Manish Jha

When you create a new datastore in vSphere, each VMFS volume is assigned a unique identifier (UUID) and this UUID info is stored in a metadata file as unique hexadecimal number.

You can see these UUID via ssh console of Esxi host.  In the /vmfs/volumes directory, each VMFS volume has a long string and a human readable names (which we configure from GUI while creating a datastore) as links to the UUID.

lun-1.PNG

The UUID is comprised of four components. Lets understand this by taking example of one of the vmfs volume’s UUID : 591ac3ec-cc6af9a9-47c5-0050560346b9

  • System Time (591ac3ec)
  • CPU Timestamp (cc6af9a9)
  • Random Number (47c5)
  • MAC Address – Management Port uplink of the host used to re-signature or create the datastore (0050560346b9)

In my example, the mac address was of the Management NIC of my first Esxi host

ln-2.PNG

When a LUN is replicated or its snapshot is taken on the storage side, the copied LUN is identical to original one. As a result, if the original LUN contains a VMFS datastore with UUID X, the copied LUN appears to contain an identical VMFS datastore, or a VMFS datastore copy, with exactly the same UUID X.

Esxi host detects a lun as being a snapshot/copy by comparing the SCSI_DiskID information returned from the storage array, with the SCSI_DiskID information stored in the LVM header.

In addition to LUN snapshotting and replication, the following storage device operations might cause ESXi to mark the existing 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

ESXi can detect the VMFS datastore copy and display it in the vSphere Web Client. You have an option of mounting the datastore copy with its original UUID or changing the UUID to re-signature the datastore.

Important info before re-signaturing:

  • Resignaturing datastore is irreversible
  • VMFS replicated copy is no longer treated as replicate
  • You can resignature spanned datastore only if the extents are online

VMFS re-signaturing via command line

1: List all detected VMFS snapshot volumes with the following:

# esxcli storage vmfs snapshot list

This will list the names of the VMFS datastores and their UUIDs.

2: Mount the volumes using below command:

# esxcli storage vmfs snapshot resignature –volume-label=<label>|–volume-uuid=<id>

Since the above commands are ran directly on an Esxi host, We need to run it on all host in the cluster if we want all host to have access to this LUN.

NOTE: The volume which you are trying to mount should be enabled for Read/Write access.

Lab ?

Unfortunately in my lab I do not have storage device with snapshot capability. I am using openfiler for shared storage and was looking for a way to create snapshot (option is there in GUI, but not working for some strange reasons)

For more info about VMFS re-signaturing, please read this document from VMware. Steps are mentioned here

Here is a video guide for vmfs re-signaturing via command line

Resignaturing VMFS Volumes via esxcli

I hope you find this post informational. Feel free to share this on social media if it is worth sharing. Be sociable 🙂

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