How to get the WWID of the disk within a RHEL guest on VMware ESX host?

Resolution

• How to identify the same disk in the hypervisor and guest
• How to use WWIDs inside udev rules to identify a disk
• VMware Hypervisor Steps to Enable Disk Identification Export to Guest
• Post Change Verification
• Common Questions

• Related:
  • How to check disk.EnableUUID parameter from VM in WMware's vSphere for Openshift Container Platform

VMware Hypervisor Steps to Enable Disk Identification Export to Guest

Disclaimer: The following information has been provided by Red Hat, but is outside the scope of the posted Service Level Agreements and support procedures. The information is provided as-is and any configuration settings or installed applications made from the information in this article could make the Operating System unsupported by Red Hat Global Support Services. The intent of this article is to provide information to accomplish the system's needs. Use of the information in this article at the user's own risk.

Cautions!

• Contact VMware technical support if these steps do not work as expected or for additional guidance..
• For more detailed instructions on how to change VMware settings, in this or later versions of the hypervisor, contact VMware technical support.
• The ability and methods used by VMware to export WWIDs for vmdk based, and other virtual device types exported to the RHEL guest, may be changed by VMware. If the verification method do not show the expected values after applying the steps shown below, then contact VMware for further support.
• The scsi_id location changed starting in RHEL7 release: it is no longer in a place where the standard PATH search will locate it. Starting in RHEL7 the location is `/usr/lib/udev/scsi_id`. This document references this path when referencing the commands within generic discussions. You will need to use version appropriate `scsi_id` syntax for other releases.

• Below steps apply on VMware ESX 4.1 version.

  • Method #1:
    • Shut down your virtual machine
    • Right-click the VM’s entry in the left-hand panel and select Edit Settings
    • Click the Options tab
    • Select the Advanced -> General item on the left and click the Configuration Parameters… button you then see displayed on the right
    • Click the Add Row button
    • Add disk.EnableUUID as the name of the new row, and the word TRUE as its value (don’t use quotation marks around either of these entries).
    • Click OK to make the new parameter addition ‘stick’.

  • Method #2: Edit guest vmx file

    • Alternatively, you may be able to achieve the same thing by editing the guest vmx file within the VMware hypervisor and adding the following line:

    disk.EnableUUID = "TRUE"
    

  • Power on or reboot the VMware guest so changes are applied

Verification

• The steps to enable disk.UUID attribute within ESX have been applied
• The VM guest was powered on or rebooted after disk.UUID enabled within ESX

• Verify WWID (disk.UUID) is being provided to RHEL guest from VMware hypervisor by using the scsi_id command. To get the WWID of say, /dev/sda, run this command:If scsi_id produces no output, contact VMware for additional ESXi configuration support.

  • For RHEL7 and RHEL8

        # /lib/udev/scsi_id --whitelisted --replace-whitespace --device=/dev/sda
        36000c2931a129f3c880b8d06ccea1b01
    

  • For RHEL6

        # scsi_id --whitelisted --replace-whitespace --device=/dev/sda
        36000c2931a129f3c880b8d06ccea1b01
    

  • For RHEL5

        #scsi_id -g -u -s /block/sdb
        36000c2931a129f3c880b8d06ccea1b01
    

How to use WWIDs within udev rules

• Using WWIDs is within udev rules is the preferred method of identifying the disk to apply an action to. Kernel assigned names such as 'sda', are not persistent across boots. A common action for a udev rule is to setup up a permanent user specified disk name for use within your system. For example the following sets up /dev/mydeviceX links to the specified device, where X is the partition number.

  • RHEL8 udev rule syntax

        KERNEL=="sd*", ENV{ID_SERIAL}=="36000c2931a129f3c880b8d06ccea1b01",  SYMLINK+="mydevice%n"
    

  • RHEL7 udev rule syntax

        KERNEL=="sd*", BUS=="scsi", PROGRAM=="/lib/udev/scsi_id --whitelisted --replace-whitespace --device=%N",    \ 
            RESULT=="36000c2931a129f3c880b8d06ccea1b01",         SYMLINK+="mydevice%n"
    

  • RHEL6 udev rule syntax

        KERNEL=="sd*", BUS=="scsi", PROGRAM=="/sbin/scsi_id --whitelisted --replace-whitespace --device=%N",    \ 
            RESULT=="36000c2931a129f3c880b8d06ccea1b01",         SYMLINK+="mydevice%n"
    

  • RHEL5 udev rule syntax

        KERNEL=="sd*", BUS=="scsi", PROGRAM=="/sbin/scsi_id -g -u -s %p",  RESULT=="36000c2931a129f3c880b8d06ccea1b01",  \
            SYMLINK+="mydevice%n"
    

How to identify the same disk in the hypervisor and guest

A common issue can arise where you need to have a unique, common, and persistent disk identifier so that you can:

• use the same identifier from a guest sdX disk within the VMware hypervisor to locate a vmdk resource that is generating media errors on the guest side,
• use the same identifier from a guest sdX disk for udev rules as sdX names are not guaranteed to be consistent across boots,
• use the same identifier from a vmdk within VMware hypervisor to locate the newly added sdX disk name on the guest side.
  • Adding and removing disks from a guest can result in changing kernel assigned sdX name. In addition, the scsi address can change also.

This might be described via something like:

We are seeing SCSI controller ID sequence differences when new SAN disks are added or removed from guest. On VMwark side the scsi id for /dev/sdn is SCSI[0:11], but on the guest is was [21:0:0:0] but changed to [22:0:0:0] and back again as the SAN disks were added/removed. How we can find exact SCSI ID in this way on server to match it with vmware disk?

The solution is, instead of using a kernel assigned identifier such as scsi address or sdX name which are non-persistent across boots, use a self-identifier that is provided from the disk itself which is persistent across reboots. In that way, no matter the order or movement of the disk physically or virtually as seen at discovery time, or as disks are added and removed, the same identifier can always be used to positively identify the device. This allows udev rules, device mounting, and other system administration tasks to always correctly specify the disk that is the target of that current admin action.

In summary:

• non-persistent: sdX and [21:0:0:0] scsi addresses are associated with the kernel's discovery order and is kernel assigned based upon that discovery order
  • with parallel discovery order, assigned sdX names, scsi addresses and the like are not guaranteed to be consistent across reboots
  • with the addition or removal of luns assigned to a guest, the discovery order and thus sdX naming assignment, can change
  • RHEL kernel assigned designations, from scsi address to sdX names are for devices only on the guest will not be the same values as seen in the hypervisor
• persistent: an identifer that is associated with a disk (including vmdk disks), the same persistent identifier for a disk will be returned no matter what the kernel assigned sdX name or scsi address is
  • a device's persistent identifier is the same within the hypervisor or guest so can be used at the search key to find the same device under both
  • a device's persistent identifier is the same across boots, so udev rules and other commands can use this persistent identifer to support applying actions to the same device each time

A disk's WWID can be used to locate the same device in either ESX or Guest.

A device's WWID (World Wide IDentifier) is the technical solution to the above problem. WWIDs are persistent. Within the VMware hypervisor control of whether a WWID is available to the guest is controlled by the disk.UUID feature. Follow the steps outlined above to enable disk.UUID within the hypervisor.

See additional information on non-persistent and persistent device names within the kernel documentation:

• RHEL 8: "Managing Storage Devices: Chapter 4. Overview of persistent naming attributes" and this may be referenced for RHEL 9 as well
• RHEL 7: "Storage Administration Guide: 25.8. Persistent Naming"
• RHEL 6: "Storage Administration Guide: Chapter 28. Persistent Naming"

Commonly Asked Questions

• Why scsi_id command is not returning disk WWID when run against VMware virtual disks?
• Running scsi_id command on a VMware virtual disk returns no output.

The disk.UUID VMware feature needs to be enabled within the hypervisor. Follow steps outlined above. This is valid for vmdk provisioned devices and other other such virtual devices supplied by VMware to the guest.

When the disk.UUID VMware feature is not enabled, then scsi_id generates no output.

# /usr/lib/udev/scsi_id --whitelisted --replace-whitespace --device=/dev/sda
#

The kernel depends on information returned from the device in order to provide a WWID or other unique identifier. In the cases of virtual disks, this information must be supplied by the VMware hypervisor.

The nominal method used within scsi_id command to obtain this information is to use the output from scsi INQUIRY command to fetch VPD (vital product data) information from either page 83 (device identification) or, if that is not available, page 80 (unit serial number). In the case of vmdk based virtual disks provided to the RHEL guest, these pages are not be available until the `disk.UUID` feature is enabled within the hypervisor. So the root cause when blank output is generated from `scsi_id` for virtual disks is the lack of appropriate VPD information being provided from the hypervisor. To correct this, enable the disk.UUID feature within the VMware hypervisor by following the [steps outlined above](#STEPS).

You can use the following commands from the `sg3_utils` package to see whether the hypervisor is providing a WWID or not.

• disk.UUID disabled:

  # sg_inq -p 0x83 /dev/sda
      VPD INQUIRY: Device Identification page
          inquiry: field in cdb illegal (page not supported)

• disk.UUID enabled:
  

  # sg_inq -p 0x83 /dev/sda
      VPD INQUIRY: Device Identification page
        Designation descriptor number 1, descriptor length: 20
          designator_type: NAA,  code_set: Binary
          associated with the Addressed logical unit
            NAA 6, IEEE Company_id: 0x0c29
           Vendor Specific Identifier: 0x31a129f3c
            Vendor Specific Identifier Extension: 0x880b8d06ccea1b01
            [0x600.0c293.1a129f3c880b8d06ccea1b01]

• Does this work on vmdk based virtual disks exported from VMware to a RHEL guest?

Yes. Enabling the disk.UUID works with vmdk devices so that the guest can obtain a WWID for sdX virtual disks.

• I need a WWID for a VMware guest disk for udev rules, how do I find it on the RHEL guest?
• How to obtain a WWID from a VMware guest (RHEL sdX disk)?

Use the scsi_id command to obtain the WWID for a sdX device:

# /usr/lib/udev/scsi_id --whitelisted --replace-whitespace --device=/dev/sda
36000c2931a129f3c880b8d06ccea1b01

If blank, verify disk.UUID is enabled within the VMware hypervisor. If problems still exist, contact VMware for assistance.

• How do I find the sdX name of a disk in the RHEL guest when I know have the allocated disk resource information from the hypervisor?
• How to find WWID of the sd devices to create UDEV rules for VMware exported disk.
• How to find WWID of the disk in a RHEL guest on the ESX host.

Using the disk.UUID from the VMware hypervisor, search each RHEL guest sdX for a match. For example:

# ls -1cv /dev/sd* | grep -v [0-9] | while read disk; do  \
echo -n "$disk " ; /usr/lib/udev/scsi_id --whitelisted --replace-whitespace --device=$disk ; done | grep 36000c2931a129f3c880b8d06ccea1b01
/dev/sda 36000c2931a129f3c880b8d06ccea1b01

If blank output for scsi_id, then verify disk.UUID feature within the VMware hypervisor is enabled. If problems still exist, contact VMware for assistance.

• How do I identify the allocated resource within the VMware hypervisor when I know the sdX name within the RHEL guest?
• How to match sd device to vmdk on the VMware host side using WWID

Get the WWID from the RHEL guest side for the sdX in question:

# /usr/lib/udev/scsi_id --whitelisted --replace-whitespace --device=/dev/sda
    36000c2931a129f3c880b8d06ccea1b01

Find the matching WWID on the VMWare hypervisor side

• From the ESX(i) console

  # vmkfstools -J getuuid /vmfs/volumes/datastore/rhel/rhel.vmdk
              60 00 c2 93 1a 12 9f 3c 88 0b 8d 06 cc ea 1b 01