Troubleshooting a converted virtual machine that experiences poor performance
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Troubleshooting a converted virtual machine that experiences poor performance


Article ID: 326188


Updated On:


VMware vCenter Server VMware vSphere ESXi


This article provides basic guidelines and suggestions to troubleshoot common causes of post-conversion performance issues.


After converting a physical machine to a virtual machine, you may experience the following symptoms:

  • Slow performance, especially when under heavy loads.
  • Virtual machine intermittently becomes unresponsive.
  • Virtual machine is slower than its physical counterpart.


VMware ESXi 3.5.x Embedded
VMware ESXi 3.5.x Installable
VMware ESXi 4.0.x Installable
VMware ESXi 4.0.x Embedded
VMware vCenter Server 5.1.x
VMware ESX Server 3.0.x
VMware vCenter Server 5.0.x
VMware vCenter Server 4.1.x
VMware VirtualCenter 2.0.x
VMware vSphere ESXi 5.0
VMware vCenter Server 4.0.x
VMware ESX Server 3.5.x
VMware ESXi 4.1.x Installable
VMware ESXi 4.1.x Embedded
VMware VirtualCenter 2.5.x
VMware ESX 4.1.x
VMware ESX 4.0.x
VMware vSphere ESXi 5.1


If you experience performance issues after converting physical machines to virtual machines, try these troubleshooting suggestions:
  • Remove any old hardware drivers and hidden devices. Hidden devices can inadvertently cause performance issues because they call on hardware that is no longer present after the conversion. They can also cause unexpected behavior because the underlying virtual hardware is different than the system from which it was converted. For more information on removing hidden devices, see the Microsoft Knowledge Base article 315539.

    Note: The preceding link was correct as of May 20, 2013. If you find the link is broken, provide feedback and a VMware employee will update the link.
  • Ensure that you are using the appropriate SCSI controller for your guest operating system. When using VMware vCenter Converter, Windows may continue to use the SCSI drivers that are in place before the conversion. These drivers are not optimal for a virtualized environment and may cause unexpected performance problems or failures. For more information about which SCSI controller to use, see the Guest Operating System Installation Guide. To change your SCSI controller, see Changing the virtual SCSI controller of a virtual machine (1002149).
  • In Windows, double-check the HAL to ensure that it is using the ACPI Multi Processor driver, especially if during the conversion you allocate more processors to the virtual machine than it has on the physical server. Windows does not automatically change the HAL to accommodate the additional vCPUs. Even if the virtual machine is configured with multiple processors, Windows ignores the other processors until the HAL driver is changed.

    To check the HAL driver:
    1. Right-click My Computer > Manage > Device Manager > Computer.
    2. Make sure that the driver listed is the ACPI Multi Processor driver.
    3. If the driver is not set to ACPI Multi Processor, see the Microsoft Knowledge Base article 309283 to correct the driver. Please be aware that the issue KB 309283 was referring to has been fixed in Windows Server 2003 Service Pack 2. KB 309283 itself was withdrawn without any replacement.
  • Remove any leftover software applications. Software applications that optimize hardware, monitor physical hardware, or interact with SCSI Controllers or video devices are not needed after conversion and can cause issues with a virtual machine.
  • Ensure that sufficient CPU and memory resources have been dedicated to the virtual machine. Applications such as Microsoft SQL and Exchange can be resource intensive and require a certain amount of resources to function properly. VMware recommends that you consult your software vendor to determine the minimum requirements.

    To configure optimal CPU and memory settings:

    Note: These steps configure common CPU and memory settings that address many resource issues. However, these settings may not be suitable for every environment. For more information about these settings, see Managing CPU Resources and Managing Memory Resources in the Resource Management Guide for your version of ESX.
    1. Configure your CPU settings to have unlimited resources:
      1. Right-click the virtual machine, and click Edit Settings.
      2. Click the Resources tab.
      3. Click CPU.
      4. Set Shares to Normal.
      5. Set Limit to 0 and select Unlimited.
    2. Configure your memory settings to have a reservation:

      Note: In some cases, setting a reservation for memory can increase performance. For example, some applications that manage their own memory (such as database servers) allocate large chunks of memory immediately. This request for memory may not be scheduled quickly enough and the virtual machine suffers from I/O disk and CPU queuing. Reserving memory in advance prevents the virtual machine from having to schedule the memory request and allows it to immediately acquire it. If you have several virtual machines that perform these operations, ensure that your ESX host can handle multiple reservations as you do not want to over-commit the RAM on the host.

      1. Click Memory.
      2. Set Shares to Normal.
      3. Set Reservation to desired number in MB.
      4. Select Unlimited.
    3. Click OK to save these settings.

Additional Information

For additional information on using VMware vCenter Converter, see: