Update documentation for strict host tags

Author: vishesh92

source/adminguide/host_and_storage_tags.rst

@@ -49,6 +49,37 @@ To explain the behavior of host tags, some examples will be demonstrated with tw
     * Offering: h3
     None of the hosts have compatible tags and it will not be possible to deploy a VM with the offering. However, CloudStack ignores this behavior when a host is manually selected.
 
+.. _strict-host-tags:
+Strict Host Tags
+-----------------
+During certain operations, like changing compute offering, and starting or live
+migrating an instance to a specific host, CloudStack will ignore the host tags,
+which can lead to issues where a instance starts on an undesired host.
+
+To avoid a mismatch of tags, tags which must always match during any operation
+can be defined in `vm.strict.host.tags`. If the tags which doesn't match are
+also part of `vm.strict.host.tags`, the operation will fail.
+
+If `resource.limit.host.tags` are defined and
+`vm.strict.resource.limit.host.tag.check` is set to true, the tags defined in
+`resource.limit.host.tags` are included with the `vm.strict.host.tags`.
+
+.. list-table:: Strict host tags related global settings
+   :header-rows: 1
+
+   * - Parameter
+     - Default
+     - Description
+   * - ``vm.strict.host.tags``
+     - empty
+     - A comma-separated list of tags which must match during operations like
+       modifying the compute offering for an instance, and starting or live
+       migrating an instance to a specific host.
+   * - ``vm.strict.resource.limit.host.tag.check``
+     - `true`
+     - If set to true, tags specified in `resource.limit.host.tags` are also
+       included in `vm.strict.host.tags`.
+
 Storage tags
 ------------
 Storage tags are responsible for directing volumes to compatible primary storage. They are validated with the storage tags entered in the disk offerings or system offerings.

source/adminguide/virtual_machines.rst

@@ -276,6 +276,14 @@ CloudStack's database.  The reboot process does not do this.
 
 When starting an Instance, admin Users have the option to specify a pod, cluster, or host.
 
+.. note::
+   When starting an instance, it's possible to specify a host for deployment,
+   even if the host's tags don't match the instance's tags. This can lead to a
+   mismatch between the VM's tags and the host's tags, which may not be
+   desirable.
+
+   To avoid this, refer to the :ref:`strict-host-tags` section
+
 
 Deleting Instance
 ------------------
@@ -441,6 +449,11 @@ Instance, you can change the Instance's compute offering.
 
 #. Click OK.
 
+.. note::
+   When changing the service offering for an instance, it's possible to have a
+   mismatch of host tags which can be problematic.
+
+   For more information on how to prevent this, see :ref:`strict-host-tags`.
 
 .. _cpu-and-memory-scaling:
 
@@ -637,6 +650,12 @@ To manually live migrate an Instance
 
       where i in [0,..,N] and N = number of volumes of the Instance
 
+.. note::
+   During live migration, there can be a mismatch between the instance's tags
+   with the destination host's tags which might be undesirable.
+
+   For more details on how to prevent this, see :ref:`strict-host-tags`.
+
 Moving Instance's Volumes Between Storage Pools (offline volume Migration)
 --------------------------------------------------------------------------