Hyper-V

Finding RDM LUN UUIDs in a vSphere Cluster | Lazy Admin Blog

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If you’re managing a large virtual environment, keeping track of Raw Device Mappings (RDMs) can be a nightmare. Unlike standard virtual disks (VMDKs) that live neatly inside a datastore, RDMs are directly mapped to a LUN on your SAN.

When your storage team asks, “Which VM is using LUN ID 55?”, you don’t want to check every VM manually. This PowerCLI script will scan your entire cluster and export a list of all RDMs along with their Canonical Name (NAA ID) and Device Name.

The PowerCLI One-Liner

This command connects to your cluster, filters for disks that are either RawPhysical (Pass-through) or RawVirtual, and spits out the details to a text file for easy searching.

Run this in your PowerCLI window:

PowerShell

Get-Cluster 'YourClusterName' | Get-VM | Get-HardDisk -DiskType "RawPhysical","RawVirtual" | Select-Object @{N="VM";E={$_.Parent.Name}},Name,DiskType,ScsiCanonicalName,DeviceName | Format-List | Out-File –FilePath C:\temp\RDM-list.txt

Breaking Down the Output

Once you open C:\temp\RDM-list.txt, here is what you are looking at:

  • Parent: The name of the Virtual Machine.
  • Name: The label of the hard disk (e.g., “Hard disk 2”).
  • DiskType: Confirms if it’s Physical (direct SCSI commands) or Virtual mode.
  • ScsiCanonicalName: The NAA ID (e.g., naa.600601...). This is the “Universal ID” your storage array uses.
  • DeviceName: The internal vSphere path to the device.

Why do you need this?

  1. Storage Migrations: If you are decommissioning a storage array, you must identify every RDM to ensure you don’t leave a “Ghost LUN” behind.
  2. Troubleshooting Performance: If a specific LUN is showing high latency on the SAN side, this script tells you exactly which VM is the “noisy neighbor.”
  3. Audit & Compliance: Great for keeping a monthly record of physical hardware mappings.

Lazy Admin Note: This script specifically uses VMware PowerCLI cmdlets (Get-HardDisk). If you are looking for similar info on a Hyper-V host, you would typically use Get-VMHardDiskDrive and look for the DiskNumber property to correlate with physical disks in Disk Management.

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Hyper-V Performance Hack: The Essential Antivirus Exclusions List | Lazy Admin Blog

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Running antivirus on your Hyper-V host is a security must, but if you don’t configure it correctly, you’re asking for trouble. We’re talking “disappearing” VMs, corrupted virtual disks, and performance so sluggish you’ll think you’re back on physical hardware from 2005.

The culprit is usually the Real-Time Scanning engine trying to “inspect” a 100GB .vhdx file every time the guest OS writes a single bit. Here is the definitive “Lazy Admin” guide to Hyper-V AV exclusions.

1. File Extension Exclusions

Tell your AV to keep its hands off these specific virtual machine file types:

  • Virtual Disks: .vhd, .vhdx
  • Snapshots/Checkpoints: .avhd, .avhdx
  • Saved State: .vsv, .bin, .vmgs
  • Configuration: .xml, .vmcx, .vmrs
  • ISO Images: .iso
  • Tracking: .rct (Resilient Change Tracking)

2. Directory Exclusions

If you are using the default paths, exclude these. If you have a dedicated D:\VMs drive (which you should!), exclude that entire custom path as well.

  • Default Configs: C:\ProgramData\Microsoft\Windows\Hyper-V
  • Default VHDs: C:\Users\Public\Documents\Hyper-V\Virtual Hard Disks
  • Default Snapshots: C:\ProgramData\Microsoft\Windows\Hyper-V\Snapshots
  • Cluster Shared Volumes (CSV): C:\ClusterStorage
  • Hyper-V Replica: Any custom replication data folders.
  • SMB 3.0 Shares: If your VMs live on a remote file server, apply these same exclusions to that file server!

Lazy Admin Pro-Tip: If you’re using a Cluster, don’t just exclude the C:\ClusterStorage folder by path. Use the Volume ID (get it via mountvol) to ensure the exclusion sticks even if drive letters or paths shift.

3. Process Exclusions

Sometimes excluding the file isn’t enough; you need to exclude the “person” opening the file. Exclude these core Hyper-V executables:

  • Vmms.exe: The Virtual Machine Management Service.
  • Vmwp.exe: The Virtual Machine Worker Process (one runs for every active VM).
  • Vmcompute.exe: (For Windows Server 2019+) The Host Compute Service.

Why this matters (The “Error 0x800704C8”)

If you don’t set these, you’ll eventually see the dreaded Error 0x800704C8 (The process cannot access the file because it is being used by another process). This happens when your AV locks the VM’s configuration file exactly when Hyper-V tries to start it.

What about Windows Defender?

Good news for the truly lazy: if you are using built-in Microsoft Defender on Windows Server, it automatically detects the Hyper-V role and applies most of these exclusions for you. However, it does not always catch your custom storage paths (like E:\MyVMs), so always double-check your work!

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