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5.3. Xen limitations

Note

All limitations in this chapter are for Red Hat Enterprise Linux 5.11 except where noted.

Xen host (dom0) limitations

  • A limit of 254 para-virtualized block devices per host exists. The total number of block devices attached to guests cannot exceed 254.

Note

There are two methods for working around the para-virtualized device limit: using phy devices (devices using the physical access mode) or using LVM on the guest.
A host has no limit to the number of phy devices it can have if it has sufficient resources.
LVM, or a similar logical partitioning tool, can be used on a block device to create additional logical partitions on a single para-virtualized block device.

Xen Para-virtualization limitations

  • For x86 guests, a maximum of 16GB memory per guest.
  • For x86_64 guests, a maximum of 168GB memory per guest.
  • A maximum of 254 devices per guest.
  • A maximum of 15 network devices per guest.

Xen full virtualization limitations

  • For x86 guests, a maximum of 16GB memory per guest.
  • A maximum of four virtualized (emulated) IDE devices per guest.
    Devices using the para-virtualized drivers for fully-virtualized guests do not have this limitation.
  • Virtualized (emulated) IDE devices are limited by the total number of loopback devices supported by the system. The default number of available loopback devices on Red Hat Enterprise Linux 5.11 is 8. That is, by default, all guests on the system can each have no more than 8 virtualized (emulated) IDE devices.
    For more information on loopback devices, their creation and use, see the Red Hat Knowledge Solution 1721.

    Note

    The number of available loopback devices can be raised by modifying the kernel limit.
    In the /etc/modprobe.conf file, add the following line:
    options loop max_loop=64
    
    Reboot the machine or run the following commands to update the kernel with this new limit:
    # rmmod loop
    # modprobe loop
    
  • A limit of 254 para-virtualized block devices per host. The total number of block devices (using the tap:aio driver) attached to guests cannot exceed 254 devices.
  • A maximum of 254 block devices using the para-virtualized drivers per guest.
  • A maximum of 15 network devices per guest.
  • A maximum of 15 virtualized SCSI devices per guest.

PCI passthrough limitations

  • PCI passthrough (attaching PCI devices to guests) is presently only supported on the following architectures:
    • 32 bit (x86) systems.
    • Intel 64 systems.
    • Intel Itanium 2 systems.