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nova
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Add admin doc information about image cache resource accounting 

This adds some details to the image cache page in the admin docs about
how image cache disk usage is (not) considered in the scheduler disk
space calculation. Workarounds and mitigation strategies are provided.

Change-Id: I7f40f167cea073a73cf249a9adfd73e1187c031b
Related-Bug: #1878024
(cherry picked from commit ab3fab0322)
This commit is contained in:
Dan Smith 2020-05-19 11:27:03 -07:00 committed by Balazs Gibizer
parent 968981b585
commit 57702ad484
1 changed files with 40 additions and 0 deletions
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doc/source/admin/image-caching.rst
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@ -51,6 +51,46 @@ documentation for the configuration options in the
need not be downloaded (and thus cached) at the compute node at
all.
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:
#. **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.
#. **Enable optional reserved disk amount behavior**. The
configuration workaround
:oslo.config:option:`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
-----------------