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Best Practices for running Linux on Hyper-V

 

Updated: August 27, 2015

Applies To: Hyper-V Server 2012, Windows 8.1, Windows Server 2008 R2, Windows Server 2012 R2, Microsoft Hyper-V Server Technical Preview, Windows Server 2012, Hyper-V Server 2012 R2, Windows 10 Technical Preview, Windows Server Technical Preview, Windows 8

This document contains a list of recommendations for running Linux virtual machine on Hyper-V.

Some Linux file systems may consume significant amounts of real disk space even when the file system is mostly empty. To reduce the amount of real disk space usage of dynamic VHDX files, consider the following recommendations:

  • When creating the VHDX, use 1MB BlockSizeBytes (from the default 32MB) in PowerShell, for example:

    PS > New-VHD –Path C:\MyVHDs\test.vhdx –SizeBytes 127GB –Dynamic –BlockSizeBytes 1MB
    
    
  • The ext4 format is preferred to ext3 because ext4 is more space efficient than ext3 when used with dynamic VHDX files.

  • When creating the filesystem specify the number of groups to be 4096, for example:

    # mkfs.ext4 –G 4096 /dev/sdX1 
    

Because of legacy hardware being removed from emulation in Generation 2 virtual machines, the grub menu countdown timer counts down too quickly for the grub menu to be displayed, immediately loading the default entry. Until grub is fixed to use the EFI-supported timer, modify /boot/grub/grub.conf, /etc/default/grub, or equivalent to have “timeout=100000” instead of the default “timeout=5”.

Because the PIT timer is not present in Generation 2 Virtual Machines, network connections to the PxE TFTP server can be prematurely terminated and prevent the bootloader from reading Grub configuration and loading a kernel from the server.

On RHEL 6.x, the legacy grub v0.97 EFI bootloader can be used instead of grub2 as described here: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Installation_Guide/s1-netboot-pxe-config-efi.html

On Linux distributions other than RHEL 6.x, similar steps can be followed to configure grub v0.97 to load Linux kernels from a PxE server.

Additionally, on RHEL/CentOS 6.6 keyboard and mouse input will not work with the pre-install kernel which prevents specifying installation options in the menu. A serial console must be configured to allow choosing installation options.

  • In the efidefault file on the PxE server, add the following kernel parameter “console=ttyS1”

  • On the VM in Hyper-V, setup a COM port using this PowerShell cmdlet:

    Set-VMComPort –VMName <Name> -Number 2 –Path \\.\pipe\dbg1
    

Specifying a kickstart file to the pre-install kernel would also avoid the need for keyboard and mouse input during installation.

Linux virtual machines that will be deployed using failover clustering should be configured with a static media access control (MAC) address for each virtual network adapter. In some versions of Linux, the networking configuration may be lost after failover because a new MAC address is assigned to the virtual network adapter. To avoid losing the network configuration, ensure that each virtual network adapter has a static MAC address. You can configure the MAC address by editing the settings of the virtual machine in Hyper-V Manager or Failover Cluster Manager.

Configure and use the virtual Ethernet adapter, which is a Hyper-V-specific network card with enhanced performance. If both legacy and Hyper-V-specific network adapters are attached to a virtual machine, the network names in the output of ifconfig –a might show random values such as _tmp12000801310. To avoid this issue, remove all legacy network adapters when using Hyper-V-specific network adapters in a Linux virtual machine.

The Linux kernel has four different I/O schedulers to reorder requests with different algorithms. NOOP is a first-in first-out queue that passes the schedule decision to be made by the hypervisor. It is recommended to use NOOP as the scheduler when running Linux virtual machine on Hyper-V. To change the scheduler for a specific device, in the boot loader’s configuration (/etc/grub.conf, for example), add elevator=noop to the kernel parameters, and then restart.

Linux virtual machines configured to use more than 7 virtual processors should add numa=off to the GRUB boot.cfg to work around a known issue in the 2.6.x Linux kernels. Linux virtual machines configured to use more than 30 GB RAM should also add numa=off to the GRUB boot.cfg.

In case the dump capture kernel ends up with a panic on boot, reserve more memory for the kernel. For example, change the parameter crashkernel=384M-:128M to crashkernel=384M-:256M in the Ubuntu grub configuration file.

Hyper-V allows shrinking virtual disk (VHD) files without regard for any partition, volume, or file system data structures that may exist on the disk. If the VHD is shrunk to where the end of the VHD comes before the end of a partition, data can be lost, that partition can become corrupted, or invalid data can be returned when the partition is read.

After resizing a VHD, administrators should use a utility like fdisk or parted to update the partition, volume, and file system structures to reflect the change in the size of the disk. Shrinking or expanding the size of a VHD that has a GUID Partition Table (GPT) will cause a warning when a partition management tool is used to check the partition layout, and the administrator will be warned to fix the first and secondary GPT headers. This manual step is safe to perform without data loss.

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