• Correct Memory Sizing for Child Partitions
  • Correct Memory Sizing for Root Partition
  • Storage I/O Performance
  • Synthetic SCSI Controller
  • Virtual Hard Disk Types
  • Disabling File Last Access Time Check
  • Performance Tuning Guidelines for Windows Server 2008 R2 April 12, 2013 Abstract

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    Memory Performance

    The hypervisor virtualizes the guest physical memory to isolate VMs from each other and provide a contiguous, zero-based memory space for each guest operating system. In general, memory virtualization can increase the CPU cost of accessing memory. On non-SLAT-based hardware, frequent modification of the virtual address space in the guest operating system can significantly increase the cost.

    Enlightened Guests

    Windows Server 2008 R2 and Windows Server 2008 include kernel enlightenments and optimizations to the memory manager to reduce the CPU overhead from Hyper-V memory virtualization. Workloads that have a large working set in memory can benefit from using Windows Server 2008 R2 or Windows Server 2008 as a guest. These enlightenments reduce the CPU cost of context switching between processes and accessing memory. Additionally, they improve the multiprocessor (MP) scalability of Windows Server guests.

    Correct Memory Sizing for Child Partitions

    You should size VM memory as you typically do for server applications on a physical machine. You must size it to reasonably handle the expected load at ordinary and peak times because insufficient memory can significantly increase response times and CPU or I/O usage.

    You can enable Dynamic Memory to allow Windows to size VM memory dynamically. The recommended initial memory size for Windows Server 2008 R2 guests is at least 512 MB. With Dynamic Memory, if applications in the VM experience launching problems, you can increase the pagefile size for the VM. To increase the VM pagefile size, navigate to Control Panel > System > Advanced System Settings > Advanced. From this tab, navigate to Performance Settings > Advanced > Virtual memory. For the Custom size selection, configure the Initial Size to the VM’s Memory Demand when VM reaches its steady state, and set the Maximum Size to three times the Initial Size. For more information about Dynamic Memory configuration, see “Resources” later in this guide.

    When running Windows in the child partition, you can use the following performance counters within a child partition to identify whether the child partition is experiencing memory pressure and is likely to perform better with a higher VM memory size:

    Performance counter

    Suggested threshold value

    Memory – Standby Cache Reserve Bytes

    Sum of Standby Cache Reserve Bytes and Free and Zero Page List Bytes should be 200 MB or more on systems with 1 GB, and 300 MB or more on systems with 2 GB or more of visible RAM.

    Memory – Free & Zero Page List Bytes

    Sum of Standby Cache Reserve Bytes and Free and Zero Page List Bytes should be 200 MB or more on systems with 1 GB, and 300 MB or more on systems with 2 GB or more of visible RAM.

    Memory – Pages Input/Sec

    Average over a 1-hour period is less than 10.

    Correct Memory Sizing for Root Partition

    The root partition must have sufficient memory to provide services such as I/O virtualization, snapshot, and management to support the child partitions. Hyper-V calculates an amount of memory known as the root reserve, which is guaranteed to be available to the root partition and never assigned to virtual machines. It is calculated automatically based on the host’s physical memory and system architecture. This logic applies for supported scenarios with no applications running in the root.

    Storage I/O Performance

    Hyper-V supports synthetic and emulated storage devices in VMs, but the synthetic devices generally can offer significantly better throughput and response times and reduced CPU overhead. The exception is if a filter driver can be loaded and reroutes I/Os to the synthetic storage device. Virtual hard disks (VHDs) can be backed by three types of VHD files or raw disks. This section describes the different options and considerations for tuning storage I/O performance.

    For more information, refer to “Performance Tuning for the Storage Subsystem” earlier in this guide, which discusses considerations for selecting and configuring storage hardware.

    Synthetic SCSI Controller

    The synthetic storage controller provides significantly better performance on storage I/Os with less CPU overhead than the emulated IDE device. The VM Integration Services include the enlightened driver for this storage device and are required for the guest operating system to detect it. The operating system disk must be mounted on the IDE device for the operating system to boot correctly, but the VM integration services load a filter driver that reroutes IDE device I/Os to the synthetic storage device.

    We strongly recommend that you mount the data drives directly to the synthetic SCSI controller because that configuration has reduced CPU overhead. You should also mount log files and the operating system paging file directly to the synthetic SCSI controller if their expected I/O rate is high.

    For highly intensive storage I/O workloads that span multiple data drives, each VHD should be attached to a separate synthetic SCSI controller for better overall performance. In addition, each VHD should be stored on separate physical disks.

    Virtual Hard Disk Types

    There are three types of VHD files. We recommend that production servers use fixed-sized VHD files for better performance and also to make sure that the virtualization server has sufficient disk space for expanding the VHD file at run time. The following are the performance characteristics and trade-offs between the three VHD types:

    Dynamically expanding VHD.

    Space for the VHD is allocated on demand. The blocks in the disk start as zeroed blocks but are not backed by any actual space in the file. Reads from such blocks return a block of zeros. When a block is first written to, the virtualization stack must allocate space within the VHD file for the block and then update the metadata. This increases the number of necessary disk I/Os for the write and increases CPU usage. Reads and writes to existing blocks incur both disk access and CPU overhead when looking up the blocks’ mapping in the metadata.

    Fixed-size VHD.

    Space for the VHD is first allocated when the VHD file is created. This type of VHD is less apt to fragment, which reduces the I/O throughput when a single I/O is split into multiple I/Os. It has the lowest CPU overhead of the three VHD types because reads and writes do not need to look up the mapping of the block.

    Differencing VHD.

    The VHD points to a parent VHD file. Any writes to blocks never written to before result in space being allocated in the VHD file, as with a dynamically expanding VHD. Reads are serviced from the VHD file if the block has been written to. Otherwise, they are serviced from the parent VHD file. In both cases, the metadata is read to determine the mapping of the block. Reads and writes to this VHD can consume more CPU and result in more I/Os than a fixed-sized VHD.
    Snapshots of a VM create a differencing VHD to store the writes to the disks since the snapshot was taken. Having only a few snapshots can elevate the CPU usage of storage I/Os, but might not noticeably affect performance except in highly I/O-intensive server workloads.

    However, having a large chain of snapshots can noticeably affect performance because reading from the VHD can require checking for the requested blocks in many differencing VHDs. Keeping snapshot chains short is important for maintaining good disk I/O performance.

    Passthrough Disks

    The VHD in a VM can be mapped directly to a physical disk or logical unit number (LUN), instead of a VHD file. The benefit is that this configuration bypasses the file system (NTFS) in the root partition, which reduces the CPU usage of storage I/O. The risk is that physical disks or LUNs can be more difficult to move between machines than VHD files.

    Large data drives can be prime candidates for passthrough disks, especially if they are I/O intensive. VMs that can be migrated between virtualization servers (such as quick migration) must also use drives that reside on a LUN of a shared storage device.

    Disabling File Last Access Time Check

    Windows Server 2003 and earlier Windows operating systems update the last-accessed time of a file when applications open, read, or write to the file. This increases the number of disk I/Os, which further increases the CPU overhead of virtualization. If applications do not use the last-accessed time on a server, system administrators should consider setting this registry key to disable these updates.


    HKLM\System\CurrentControlSet\Control\FileSystem\ (REG_DWORD)

    By default, Windows Server 2008 R2 disables the last-access time updates.

    Physical Disk Topology

    VHDs that I/O-intensive VMs use generally should not be placed on the same physical disks because this can cause the disks to become a bottleneck. If possible, they should also not be placed on the same physical disks that the root partition uses. For a discussion on capacity planning for storage hardware and RAID selection, see “Performance Tuning for the Storage Subsystem” earlier in this guide.

    I/O Balancer Controls

    The virtualization stack balances storage I/O streams from different VMs so that each VM has similar I/O response times when the system’s I/O bandwidth is saturated. The following registry keys can be used to adjust the balancing algorithm, but the virtualization stack tries to fully use the I/O device’s throughput while providing reasonable balance. The first path should be used for storage scenarios, and the second path should be used for networking scenarios:

    HKLM\System\CurrentControlSet\Services\StorVsp\ = (REG_DWORD)

    HKLM\System\CurrentControlSet\Services\VmSwitch\ = (REG_DWORD)
    Both storage and networking have three registry keys at the preceding StorVsp and VmSwitch paths, respectively. Each value is a DWORD and operates as follows. We do not recommend this advanced tuning option unless you have a specific reason to use it. Note that these registry keys might be removed in future releases:


    The balancer is enabled when set to a nonzero value and disabled when set to 0. The default is enabled for storage and disabled for networking. Enabling the balancing for networking can add significant CPU overhead in some scenarios.


    This controls how much work, represented by a latency value, the balancer allows to be issued to the hardware before throttling to provide better balance. The default is 83 ms for storage and 2 ms for networking. Lowering this value can improve balance but will reduce some throughput. Lowering it too much significantly affects overall throughput. Storage systems with high throughput and high latencies can show added overall throughput with a higher value for this parameter.


    This controls how much work the balancer issues from a VM before switching to another VM. This setting is primarily for storage where finely interleaving I/Os from different VMs can increase the number of disk seeks. The default is 8 percent for both storage and networking.

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    Performance Tuning Guidelines for Windows Server 2008 R2 April 12, 2013 Abstract

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