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

doc/source/admin/image-caching.rst

@@ -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
 -----------------