Virtualization
EVC Mode & CPU Compatibility: The “Lazy Admin” FAQ
You’ve just unboxed a shiny new host with the latest Intel or AMD processor, but your current cluster is running hardware from three years ago. You try to vMotion a VM, and vSphere gives you the dreaded “CPU Incompatibility” error.
Enter Enhanced vMotion Compatibility (EVC). Here’s everything you need to know to get your mixed-hardware cluster working without the headache.
What exactly is EVC?
Think of EVC as a “lowest common denominator” filter for your CPUs. It masks the advanced features of newer processors so that every host in the cluster appears to have the exact same instruction set. This allows VMs to live-migrate between old and new hardware because the “view” of the CPU never changes.
Quick FAQ
Q: Can I mix Intel and AMD in the same EVC cluster? A: No. EVC only works within a single vendor family. You can mix different generations of Intel, or different generations of AMD, but you cannot vMotion between the two brands.
Q: Will EVC slow down my new servers? A: Technically, yes—but rarely in a way you’ll notice. It hides new instructions (like specialized encryption or AI math sets), but the raw clock speed and core count of your new CPUs are still fully utilized. Most general-purpose VMs don’t use the high-end instructions being masked.
Q: Do I need to power off VMs to enable EVC? A: It depends:
- Enabling on an empty cluster: No downtime.
- Enabling on a cluster where VMs are already running on the oldest host: Usually no downtime.
- Enabling on a cluster where VMs are running on newer hosts: You must power off those VMs so they can “re-boot” with the masked CPU instructions.
Q: What is “Per-VM EVC”? A: Introduced in vSphere 6.7, this allows you to set the EVC mode on the VM itself rather than the whole cluster. This is a lifesaver for migrating VMs across different vCenters or into the Cloud (like AWS/Azure).
How to Find Your Correct EVC Mode
Don’t guess. Use the official tool:
- Go to the VMware Compatibility Guide (CPU/EVC Matrix).
- Select your ESXi version.
- Select the CPU models of your oldest and newest hosts.
- The tool will tell you the highest supported “Baseline” you can use.
Step-by-Step: Enabling EVC on an Existing Cluster
- Select your Cluster in vCenter.
- Go to Configure > VMware EVC.
- Click Edit.
- Select Enable EVC for Intel/AMD hosts.
- Choose the Baseline that matches your oldest host.
- Validation: vCenter will check if any running VMs are currently using features above that baseline. If they are, you’ll need to shut them down before you can save the settings.
Summary Table: EVC Baselines
| If your oldest host is… | Use this EVC Mode |
| Intel Ice Lake | Intel “Ice Lake” Generation |
| Intel Cascade Lake | Intel “Cascade Lake” Generation |
| AMD EPYC Rome | AMD EPYC “Rome” Generation |
Lost Your VM? How to Find Its ESXi Host from the Guest OS
It’s a classic “Ghost in the Machine” scenario: You can RDP or SSH into a virtual machine, but you can’t find it in vCenter. Maybe it’s a massive environment with thousands of VMs, maybe the naming convention doesn’t match, or maybe you’re dealing with a rogue host that isn’t even in your main cluster.
If VMware Tools is installed and running, the VM actually knows exactly where it lives. You just have to ask it nicely through the Command Prompt.
The Magic Tool: vmtoolsd.exe
On Windows VMs, the VMware Tools service includes a CLI utility called vmtoolsd.exe. This tool can query the hypervisor for specific environment variables that are passed down to the guest.
1. Find the ESXi Hostname
If you need to know which physical server is currently crunching the cycles for your VM, run this command:
"C:\Program Files\VMware\VMware Tools\vmtoolsd.exe" --cmd "info-get guestinfo.hypervisor.hostname"2. Get the ESXi Build Details
Need to know if the underlying host is patched or running an ancient version of ESXi? Query the build number:
"C:\Program Files\VMware\VMware Tools\vmtoolsd.exe" --cmd "info-get guestinfo.hypervisor.build"Why is this useful?
- vCenter Search is failing: Sometimes the inventory search index gets corrupted, and “Name contains” returns nothing.
- Nested Environments: If you are running VMs inside VMs, this helps you verify which layer of the onion you are currently on.
- Troubleshooting Performance: If a VM is lagging, you can quickly identify the host to check for hardware alerts or CPU contention without leaving the OS.
What if I’m on Linux?
The same logic applies! Most modern Linux distributions use open-vm-tools. You can run the same query via the terminal:
vmtoolsd --cmd "info-get guestinfo.hypervisor.hostname"Important Requirement: Guest RPC
For these commands to work, the VM must have VMware Tools installed and the guestinfo variables must be accessible. In some hardened environments, admins might disable these RPC (Remote Procedure Call) queries in the .vmx file for security reasons, but in 95% of standard builds, this will work out of the box.
Hyper-V Performance Hack: The Essential Antivirus Exclusions List
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:\ClusterStoragefolder by path. Use the Volume ID (get it viamountvol) 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!
Script: Finding RDM LUN UUIDs in a vSphere Cluster
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.txtBreaking 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?
- Storage Migrations: If you are decommissioning a storage array, you must identify every RDM to ensure you don’t leave a “Ghost LUN” behind.
- 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.”
- 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 useGet-VMHardDiskDriveand look for theDiskNumberproperty to correlate with physical disks inDisk Management.
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