Adapt the admin guide to describe the direct RBD export

This documents how to set up nova and glance so the feature
for direct download from Ceph can be used.

Change-Id: I07509c67c65e988fe5149b625007e90e68488cfd

1 files changed, 30 insertions, 0 deletions

diff --git a/doc/source/admin/configuration/hypervisor-kvm.rst b/doc/source/admin/configuration/hypervisor-kvm.rst
index 243e7eb238..31a9731b15 100644
--- a/doc/source/admin/configuration/hypervisor-kvm.rst
+++ b/doc/source/admin/configuration/hypervisor-kvm.rst
@@ -260,6 +260,36 @@ Local `LVM volumes
 used. Set the :oslo.config:option:`libvirt.images_volume_group` configuration
 option to the name of the LVM group you have created.
 
+Direct download of images from Ceph
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When the Glance image service is set up with the Ceph backend and Nova is using
+a local ephemeral store (``[libvirt]/images_type!=rbd``), it is possible to
+configure Nova to download images directly into the local compute image cache.
+
+With the following configuration, images are downloaded using the RBD export
+command instead of using the Glance HTTP API. In some situations, especially
+for very large images, this could be substantially faster and can improve the
+boot times of instances.
+
+On the Glance API node in ``glance-api.conf``:
+
+.. code-block:: ini
+
+   [DEFAULT]
+   show_image_direct_url=true
+
+On the Nova compute node in nova.conf:
+
+.. code-block:: ini
+
+   [glance]
+   enable_rbd_download=true
+   rbd_user=glance
+   rbd_pool=images
+   rbd_ceph_conf=/etc/ceph/ceph.conf
+   rbd_connect_timeout=5
+
 Specify the CPU model of KVM guests
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~