Amend "Configure max number of volumes to attach" spec

Update the spec to reflect what was actually implemented.

TL;DR is during implementation, we found that the old limit in the
libvirt driver of 26 was a limit on the maximum number of disk
devices allowed to attach to a single instance, including the root
disk (and any other disks). So "volumes" wasn't really correct for
representing what is being limited and the terminology was changed
to "disk devices".

Related to blueprint conf-max-attach-volumes

Change-Id: I3152d0ed64709495ff7f13ff1d75ce62558a8731

specs/stein/implemented/conf-max-attach-volumes.rst

@@ -41,34 +41,44 @@ Use Cases
 Proposed change
 ===============
 
-When a user attempts to attach more than 26 volumes with the libvirt driver,
-the attach fails in the ``reserve_block_device_name`` method in nova-compute,
-which is eventually called by the ``attach_volume`` method in nova-api. The
-``reserve_block_device_name`` method calls
+When a user attempts to attach more than 26 disk devices with the libvirt
+driver, the attach fails in the ``reserve_block_device_name`` method in
+nova-compute, which is eventually called by the ``attach_volume`` method in
+nova-api. The ``reserve_block_device_name`` method calls
 ``self.driver.get_device_name_for_instance`` to get the next available device
 name for attaching the volume. If the driver has implemented the method, this
-is where an attempt to go beyond the maximum allowed number of volumes to
-attach, will fail. The libvirt driver fails after 26 volumes have been
+is where an attempt to go beyond the maximum allowed number of disk devices to
+attach, will fail. The libvirt driver fails after 26 disk devices have been
 attached. Drivers that have not implemented ``get_device_name_for_instance``
-appear to have no limit on the maximum number of volumes. The default
+appear to have no limit on the maximum number of disk devices. The default
 implementation of ``get_device_name_for_instance`` is located in the
 ``nova.compute.utils`` module. Only the libvirt driver has provided its own
 implementation of ``get_device_name_for_instance``.
 
 The ``reserve_block_device_name`` method is a synchronous RPC call (not cast).
 This means we can have the configured allowed maximum set differently per
-nova-compute and still fail fast in the API if the maximum has been exceeded.
+nova-compute and still fail fast in the API if the maximum has been exceeded
+during an attach volume request.
 
-We propose to add a new configuration option ``[compute]max_volumes_to_attach``
-IntOpt to use to configure the maximum allowed volumes to attach to a single
-instance per nova-compute. This way, operators can set it appropriately
-depending on what virt driver they are running and what their deployed
-environment is like. The default will be unlimited (-1) to keep the current
-behavior for all drivers except the libvirt driver.
+For a server create, rebuild, evacuate, unshelve, or live migrate request, if
+the maximum has been exceeded, the server will go into the ``ERROR`` state and
+the server fault message will indicate the failure reason.
+
+Note that the limit in the libvirt driver is actually on the total number of
+disk devices allowed to attach to a single instance including the root disk
+and any other disks. It does not differentiate between volumes and other disks.
+
+We propose to add a new configuration option
+``[compute]max_disk_devices_to_attach`` IntOpt to use to configure the maximum
+allowed disk devices to attach to a single instance per nova-compute. This way,
+operators can set it appropriately depending on what virt driver they are
+running and what their deployed environment is like. The default will be
+unlimited (-1) to keep the current behavior for all drivers except the libvirt
+driver.
 
 The configuration option will be enforced in the
 ``get_device_name_for_instance`` methods, using the count of the number of
-already attached volumes. Upon failure, an exception will be propagated to
+already attached disk devices. Upon failure, an exception will be propagated to
 nova-api via the synchronous RPC call to nova-compute, and the user will
 receive a 403 error (as opposed to the current 500 error).
 
@@ -76,7 +86,7 @@ Alternatives
 ------------
 
 Other ways we could solve this include: choosing a new hard-coded maximum only
-for the libvirt driver or creating a new quota limit for "maximum volumes
+for the libvirt driver or creating a new quota limit for "maximum disk devices
 allowed to attach" (see the ML thread in `References`_).
 
 Data model impact
@@ -112,9 +122,9 @@ None
 Other deployer impact
 ---------------------
 
-Deployers will be able to set the ``[compute]max_volumes_to_attach``
-configuration option to control how many volumes are allowed to be attached
-to a single instance per nova-compute in their deployment.
+Deployers will be able to set the ``[compute]max_disk_devices_to_attach``
+configuration option to control how many disk devices are allowed to be
+attached to a single instance per nova-compute in their deployment.
 
 Developer impact
 ----------------
@@ -142,13 +152,15 @@ Other contributors:
 Work Items
 ----------
 
-* Add a new configuration option ``[compute]max_volumes_to_attach``, IntOpt
+* Add a new configuration option ``[compute]max_disk_devices_to_attach``,
+  IntOpt
 * Modify (or remove) the libvirt driver's implementation of the
-  ``get_device_name_for_instance`` method to accomodate more than 26 volumes
-* Add enforcement of ``[compute]max_volumes_to_attach`` to the
+  ``get_device_name_for_instance`` method to accomodate more than 26 disk
+  devices
+* Add enforcement of ``[compute]max_disk_devices_to_attach`` to the
   ``get_device_name_for_instance`` methods
 * Add handling of the raised exception in the API to translate to a 403 to the
-  user, if the maximum number of allowed volumes is exceeded
+  user, if the maximum number of allowed disk devices is exceeded
 
 
 Dependencies