Nova supports caching base images on compute nodes when using a supported
virt driver.

What is Image Caching?

In order to understand what image caching is and why it is
beneficial, it helps to be familiar with the process by which an
instance is booted from a given base image. When a new instance is
created on a compute node, the following general steps are performed by
the compute manager in conjunction with the virt driver:

1. Download the base image from glance
2. Copy or COW the base image to create a new root disk image for the
   instance
3. Boot the instance using the new root disk image

The first step involves downloading the entire base image to the
local disk on the compute node, which could involve many gigabytes of
network traffic, storage, and many minutes of latency between the start
of the boot process and actually running the instance. When the virt
driver supports image caching, step #1 above may be skipped if the base
image is already present on the compute node. This is most often the
case when another instance has been booted on that node from the same
base image recently. If present, the download operation can be skipped,
which greatly reduces the time-to-boot for the second and subsequent
instances that use the same base image, as well as avoids load on the
glance server and the network connection.

By default, the compute node will periodically scan the images it has
cached, looking for base images that are not used by any instances on
the node that are older than a configured lifetime (24 hours by
default). Those unused images are deleted from the cache directory until
they are needed again.

For more information about configuring image cache behavior, see the
documentation for the configuration options in the image_cache
group.

Image Caching Resource
Accounting

Generally the size of the image cache is not part of the data Nova
includes when reporting available or consumed disk space. This means
that when nova-compute reports 100G of total disk space,
the scheduler will assume that 100G of instances may be placed there.
Usually disk is the most plentiful resource and thus the last to be
exhausted, so this is often not problematic. However, if many instances
are booted from distinct images, all of which need to be cached in
addition to the disk space used by the instances themselves, Nova may
overcommit the disk unintentionally by failing to consider the size of
the image cache.

There are two approaches to addressing this situation:

1. Mount the image cache as a separate filesystem.
   This will cause Nova to report the amount of disk space available purely
   to instances, independent of how much is consumed by the cache. Nova
   will continue to disregard the size of the image cache and, if the cache
   space is exhausted, builds will fail. However, available disk space for
   instances will be correctly reported by nova-compute and
   accurately considered by the scheduler.
2. Enable optional reserved disk amount behavior. The
   configuration workaround workarounds.reserve_disk_resource_for_image_cache
   will cause nova-compute to periodically update the reserved
   disk amount to include the statically configured value, as well as the
   amount currently consumed by the image cache. This will cause the
   scheduler to see the available disk space decrease as the image cache
   grows. This is not updated synchronously and thus is not a perfect
   solution, but should vastly increase the scheduler’s visibility
   resulting in better decisions. (Note this solution is currently
   libvirt-specific)

As above, not all backends and virt drivers use image caching, and
thus a third option may be to consider alternative infrastructure to
eliminate this problem altogether.

Image pre-caching

It may be beneficial to pre-cache images on compute nodes in order to
achieve low time-to-boot latency for new instances immediately. This is
often useful when rolling out a new version of an application where
downtime is important and having the new images already available on the
compute nodes is critical.

Nova provides (since the Ussuri release) a mechanism to request that
images be cached without having to boot an actual instance on a node.
This best-effort service operates at the host aggregate level in order
to provide an efficient way to indicate that a large number of computes
should receive a given set of images. If the computes that should
pre-cache an image are not already in a defined host aggregate, that
must be done first.

For information on how to perform aggregate-based image pre-caching,
see the image-caching-aggregates section of the Host
aggregates documentation.