118 lines
3.8 KiB
ReStructuredText
118 lines
3.8 KiB
ReStructuredText
==================
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File-backed memory
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==================
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.. important::
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As of the 18.0.0 Rocky release, the functionality described below is
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only supported by the libvirt/KVM driver.
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The file-backed memory feature in Openstack allows a Nova node to serve guest
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memory from a file backing store. This mechanism uses the libvirt file memory
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source, causing guest instance memory to be allocated as files within the
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libvirt memory backing directory.
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Since instance performance will be related to the speed of the backing store,
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this feature works best when used with very fast block devices or virtual file
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systems - such as flash or RAM devices.
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When configured, ``nova-compute`` will report the capacity configured for
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file-backed memory to placement in place of the total system memory capacity.
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This allows the node to run more instances than would normally fit
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within system memory.
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When available in libvirt and qemu, instance memory will be discarded by qemu
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at shutdown by calling ``madvise(MADV_REMOVE)``, to avoid flushing any dirty
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memory to the backing store on exit.
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To enable file-backed memory, follow the steps below:
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#. `Configure the backing store`_
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#. `Configure Nova Compute for file-backed memory`_
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.. important::
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It is not possible to live migrate from a node running a version of
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OpenStack that does not support file-backed memory to a node with file
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backed memory enabled. It is recommended that all Nova compute nodes are
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upgraded to Rocky before enabling file-backed memory.
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Prerequisites and Limitations
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Libvirt
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File-backed memory requires libvirt version 4.0.0 or newer. Discard
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capability requires libvirt version 4.4.0 or newer.
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Qemu
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File-backed memory requires qemu version 2.6.0 or newer.Discard capability
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requires qemu version 2.10.0 or newer.
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Memory overcommit
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File-backed memory is not compatible with memory overcommit.
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``ram_allocation_ratio`` must be set to ``1.0`` in ``nova.conf``, and the
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host must not be added to a :doc:`host aggregate </admin/aggregates>`
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with ``ram_allocation_ratio`` set to anything but ``1.0``.
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Huge pages
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File-backed memory is not compatible with huge pages. Instances with huge
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pages configured will not start on a host with file-backed memory enabled. It
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is recommended to use host aggregates to ensure instances configured for
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huge pages are not placed on hosts with file-backed memory configured.
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Handling these limitations could be optimized with a scheduler filter in the
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future.
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Configure the backing store
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. note::
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``/dev/sdb`` and the ``ext4`` filesystem are used here as an example. This
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will differ between environments.
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.. note::
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``/var/lib/libvirt/qemu/ram`` is the default location. The value can be
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set via ``memory_backing_dir`` in ``/etc/libvirt/qemu.conf``, and the
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mountpoint must match the value configured there.
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By default, Libvirt with qemu/KVM allocates memory within
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``/var/lib/libvirt/qemu/ram/``. To utilize this, you need to have the backing
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store mounted at (or above) this location.
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#. Create a filesystem on the backing device
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.. code-block:: console
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# mkfs.ext4 /dev/sdb
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#. Mount the backing device
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Add the backing device to ``/etc/fstab`` for automatic mounting to
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``/var/lib/libvirt/qemu/ram``
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Mount the device
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.. code-block:: console
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# mount /dev/sdb /var/lib/libvirt/qemu/ram
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Configure Nova Compute for file-backed memory
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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#. Enable File-backed memory in ``nova-compute``
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Configure Nova to utilize file-backed memory with the capacity of the
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backing store in MiB. 1048576 MiB (1 TiB) is used in this example.
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Edit ``/etc/nova/nova.conf``
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.. code-block:: ini
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[libvirt]
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file_backed_memory=1048576
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#. Restart the ``nova-compute`` service
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