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Snapshot LUN detection in ESX and ESXi (1011385)

Purpose

This article highlights the differences in snapshot LUN detection between ESX 3.x and ESX/ESXi 4.x and 5.x.

Resolution

How a LUN is detected as a snapshot in ESX 3.0.x
 
When an ESX 3.x server finds a VMFS-3 LUN, it compares the SCSI_DiskID information returned from the storage array with the SCSI_DiskID information stored in the LVM Header.
 
If the two IDs do not match, the VMFS-3 volume is not mounted.
 
A VMFS volume on ESX 3.x can be detected as a snapshot for a number of reasons:
  • LUN ID change
  • SCSI version supported by array changed (firmware upgrade)
  • Identifier type changed – Unit Serial Number vs NAA ID
How a LUN is detected as a snapshot in ESX/ESXi 3.5.x and later.
 
When trying to determine if a device is a snapshot in ESX/ESXi 3.5 and later releases, the ESX/ESXi host uses a globally unique identifier to identify each LUN, typically the Network Addressing Authority (NAA) ID.
 
NAA IDs are unique and are persistent across reboots.
 
There are many different globally unique identifiers. If the LUN does not support any of these globally unique identifiers, ESX falls back to the serial number + LUN ID used in ESX 3.x.
 

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