Extension of storage reporting design doc

This patch rewrites and extends the design doc about storage reporting
with respect to disk templates and storage types. In constrast to the
previous version, we now consider disk templates as the user-facing
entity, that the user can dis/enable for the cluster. Storage types
on the other hand describe the underlying technology used by the various
disk templates. Storage reporting will use a mapping from disk templates
to storage types to pick the correct method to report the storage for
the respective disk templates.

Note that the design doc in this state still contains some questions and
FIXMEs. Feel free to comment on those. I will complete them directly or
in future patches.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

doc/design-storagespace.rst

@@ -7,66 +7,173 @@ Storage free space reporting
 Background
 ==========
 
-Currently Space reporting is broken for all storage types except drbd or
-lvm (plain). This design looks at the root causes and proposes a way to
-fix it.
+Currently, there is no consistent management of different variants of storage
+in Ganeti. One direct consequence is that storage space reporting is currently
+broken for all storage that is not based on lvm technolgy. This design looks at
+the root causes and proposes a way to fix it.
+
+FIXME: rename the design doc to make clear that space reporting is not the only
+thing covered here?
 
 Proposed changes
 ================
 
-The changes below will streamline Ganeti to properly support
-interaction with different storage types.
+We propose to streamline handling of different storage types and disk templates.
+Currently, there is no consistent implementation for dis/enabling of disk
+templates and/or storage types.
+
+Our idea is to introduce a list of enabled disk templates, which can be
+used by instances in the cluster. Based on this list, we want to provide
+storage reporting mechanisms for the available disk templates. Since some
+disk templates share the same underlying storage technology (for example
+``drbd`` and ``plain`` are based on ``lvm``), we map disk templates to storage
+types and implement storage space reporting for each storage type.
 
 Configuration changes
 ---------------------
 
-Add a new attribute "enabled_storage_types" (type: list of strings) to the
-cluster config which holds the types of storages, for example, "file", "rados",
-or "ext". We consider the first one of the list as the default type.
+Add a new attribute "enabled_disk_templates" (type: list of strings) to the
+cluster config which holds disk templates, for example, "drbd", "file",
+or "ext". This attribute represents the list of disk templates that are enabled
+cluster-wide for usage by the instances. It will not be possible to create
+instances with a disk template that is not enabled, as well as it will not be
+possible to remove a disk template from the list if there are still instances
+using it.
+
+The list of enabled disk templates can contain any non-empty subset of
+the currently implemented disk templates: ``blockdev``, ``diskless``, ``drbd``,
+``ext``, ``file``, ``plain``, ``rbd``, and ``sharedfile``. See
+``DISK_TEMPLATES`` in ``constants.py``.
+
+Note that the abovementioned list of enabled disk types is just a "mechanism"
+parameter that defines which disk templates the cluster can use. Further
+filtering about what's allowed can go in the ipolicy, which is not covered in
+this design doc. Note that it is possible to force an instance to use a disk
+template that is not allowed by the ipolicy. This is not possible if the
+template is not enabled by the cluster.
+
+FIXME: In what way should verification between the enabled disk templates in
+the cluster and in the ipolicy take place?
+
+We consider the first disk template in the list to be the default template for
+instance creation and storage reporting. This will remove the need to specify
+the disk template with ``-t`` on instance creation.
+
+Currently, cluster-wide dis/enabling of disk templates is not implemented
+consistently. ``lvm`` based disk templates are enabled by specifying a volume
+group name on cluster initialization and can only be disabled by explicitly
+using the option ``--no-lvm-storage``. This will be replaced by adding/removing
+``drbd`` and ``plain`` from the set of enabled disk templates.
+
+Up till now, file storage and shared file storage could be dis/enabled at
+``./configure`` time. This will also be replaced by adding/removing the
+respective disk templates from the set of enabled disk templates.
+
+There is currently no possibility to dis/enable the disk templates
+``diskless``, ``blockdev``, ``ext``, and ``rdb``. By introducing the set of
+enabled disk templates, we will require these disk templates to be explicitely
+enabled in order to be used. The idea is that the administrator of the cluster
+can tailor the cluster configuration to what is actually needed in the cluster.
+There is hope that this will lead to cleaner code, better performance and fewer
+bugs.
+
+When upgrading the configuration from a version that did not have the list
+of enabled disk templates, we have to decide which disk templates are enabled
+based on the current configuration of the cluster. We propose the following
+update logic to be implemented in the online update of the config in
+the ``Cluster`` class in ``objects.py``:
+- If a ``volume_group_name`` is existing, then enable ``drbd`` and ``plain``.
+(TODO: can we narrow that down further?)
+- If ``file`` or ``sharedfile`` was enabled at configure time, add the
+respective disk template to the list of enabled disk templates.
+- For disk templates ``diskless``, ``blockdev``, ``ext``, and ``rbd``, we
+inspect the current cluster configuration regarding whether or not there
+are instances that use one of those disk templates. We will add only those
+that are currently in use.
+The order in which the list of enabled disk templates is built up will be
+determined by a preference order based on when in the history of Ganeti the
+disk templates were introduced (thus being a heuristic for which are used
+more than others).
+
+The list of enabled disk templates can be specified on cluster initialization
+with ``gnt-cluster init`` using the optional parameter
+``--enabled-disk-templates``. If it is not set, it will be set to a default
+set of enabled disk templates, which includes the following disk templates:
+``drbd`` and ``plain``. The list can be shrunk or extended by
+``gnt-cluster modify`` using the same parameter.
+
+Storage reporting
+-----------------
 
-For file storage, we'll report the storage space on the file storage dir,
-which is currently limited to one directory. In the future, if we'll have
+The storage reporting in ``gnt-node list`` will be the first user of the
+newly introduced list of enabled disk templates. Currently, storage reporting
+works only for lvm-based storage. We want to extend that and report storage
+for the enabled disk templates. The default of ``gnt-node list`` will only
+report on storage of the default disk template (the first in the list of enabled
+disk templates). One can explicitly ask for storage reporting on the other
+enabled disk templates with the ``-o`` option.
+
+Some of the currently implemented disk templates share the same base storage
+technology. Since the storage reporting is based on the underlying technology
+rather than on the user-facing disk templates, we introduce storage types to
+represent the underlying technology. There will be a mapping from disk templates
+to storage types, which will be used by the storage reporting backend to pick
+the right method for estimating the storage for the different disk templates.
+
+The proposed storage types are ``blockdev``, ``diskless``, ``ext``, ``file``,
+``lvm-pv``, ``lvm-vg``, ``rados``.
+
+The mapping from disk templates to storage types will be: ``drbd`` and ``plain``
+to ``lvm-vg``, ``file`` and ``sharedfile`` to ``file``, and all others to their
+obvious counterparts.
+
+Note that there is no disk template mapping to ``lvm-pv``, because this storage
+type is currently only used to enable the user to mark it as (un)allocatable.
+(See ``man gnt-node``.) It is not possible to create an instance on a storage
+unit that is of type ``lvm-pv`` directly, therefore it is not included in the
+mapping.
+
+The storage reporting for file storage will report space on the file storage
+dir, which is currently limited to one directory. In the future, if we'll have
 support for more directories, or for per-nodegroup directories this can be
 changed.
 
-Note that the abovementioned enabled_storage_types are just "mechanisms"
-parameters that define which storage types the cluster can use. Further
-filtering about what's allowed can go in the ipolicy, but these changes are
-not covered in this design doc.
-
-Since the ipolicy currently has a list of enabled storage types, we'll
-use that to decide which storage type is the default, and to self-select
-it for new instance creations, and reporting.
-
-Enabling/disabling of storage types at ``./configure`` time will be
-eventually removed.
+For now, we will implement only the storage reporting for non-shared storage,
+that is disk templates ``file``, ``lvm``, and ``drbd``. For disk template
+``diskless``, there is obviously nothing to report about. When implementing
+storage reporting for file, we can also use it for ``sharedfile``, since it
+uses the same file system mechanisms to determine the free space. In the
+future, we can optimize storage reporting for shared storage by not querying
+all nodes that use a common shared file for the same space information.
+
+In the future, we extend storage reporting for shared storage types like
+``rados`` and ``ext``. Note that it will not make sense to query each node for
+storage reporting on a storage unit that is used by several nodes.
+
+We will not implement storage reporting for the ``blockdev`` disk template,
+because block devices are always adopted after being provided by the system
+administrator, thus coming from outside Ganeti. There is no point in storage
+reporting for block devices, because Ganeti will never try to allocate storage
+inside a block device.
 
 RPC changes
 -----------
 
 The noded RPC call that reports node storage space will be changed to
-accept a list of <type>,<key> string tuples. For each of them, it will
+accept a list of <disktemplate>,<key> string tuples. For each of them, it will
 report the free amount of storage space found on storage <key> as known
-by the requested storage type types. For example types are ``lvm``,
-``filesystem``, or ``rados``, and the key would be a volume group name, in
-the case of lvm, a directory name for the filesystem and a rados pool name
-for rados storage.
-
-For now, we will implement only the storage reporting for non-shared storage,
-that is ``filesystem`` and ``lvm``. For shared storage types like ``rados``
-and ``ext`` we will not implement a free space calculation, because it does
-not make sense to query each node for the free space of a commonly used
-storage.
+by the requested disk template. Depending on the disk template, the key would
+be a volume group name, in case of lvm-based disk templates, a directory name
+for the file and shared file storage, and a rados pool name for rados storage.
 
-Masterd will know (through a constant map) which storage type uses which
-type for storage calculation (i.e. ``plain`` and ``drbd`` use ``lvm``,
-``file`` uses ``filesystem``, etc) and query the one needed (or all of the
-needed ones).
+Masterd will know through the mapping of disk templates to storage types which
+storage type uses which mechanism for storage calculation and invoke only the
+needed ones.
 
-Note that for file and sharedfile the node knows which directories are
-allowed and won't allow any other directory to be queried for security
-reasons. The actual path still needs to be passed to distinguish the
-two, as the type will be the same for both.
+Note that for file and sharedfile the node knows which directories are allowed
+and won't allow any other directory to be queried for security reasons. The
+actual path still needs to be passed to distinguish the two, as the type will
+be the same for both.
 
 These calculations will be implemented in the node storage system
 (currently lib/storage.py) but querying will still happen through the
@@ -75,9 +182,9 @@ These calculations will be implemented in the node storage system
 Ganeti reporting
 ----------------
 
-`gnt-node list`` can be queried for the different storage types, if they
+`gnt-node list`` can be queried for the different disk templates, if they
 are enabled. By default, it will just report information about the default
-storage type. Examples::
+disk template. Examples::
 
   > gnt-node list
   Node                       DTotal DFree MTotal MNode MFree Pinst Sinst
@@ -85,10 +192,10 @@ storage type. Examples::
   mynode2                      3.6T  3.6T  64.0G 1023M 62.0G     2     1
   mynode3                      3.6T  3.6T  64.0G 1023M 62.3G     0     2
 
-  > gnt-node list -o dtotal/lvm,dfree/rados
-  Node      DTotal (Lvm, myvg) DFree (Rados, myrados)
-  mynode1                 3.6T                      -
-  mynode2                 3.6T                      -
+  > gnt-node list -o dtotal/drbd,dfree/file
+  Node      DTotal (drbd, myvg) DFree (file, mydir)
+  mynode1                 3.6T                    -
+  mynode2                 3.6T                    -
 
 Note that for drbd, we only report the space of the vg and only if it was not
 renamed to something different than the default volume group name. With this
@@ -97,15 +204,18 @@ restrict the design here to make the transition to storage pools easier (as it
 is an interim state only). It is the administrator's responsibility to ensure
 that there is enough space for the meta volume group.
 
-When storage pools are implemented, we switch from referencing the storage
-type to referencing the storage pool name. For that, of course, the pool
-names need to be unique over all storage types. For drbd, we will use the
-default 'lvm' storage pool and possibly a second lvm-based storage pool for
-the metavg. It will be possible to rename storage pools (thus also the default
-lvm storage pool). There will be new functionality to ask about what storage
-pools are available and of what type.
-
-``gnt-cluster info`` will report which storage types are enabled, i.e.
+When storage pools are implemented, we switch from referencing the disk template
+to referencing the storage pool name. For that, of course, the pool names need
+to be unique over all storage types. For drbd, we will use the default 'drbd'
+storage pool and possibly a second lvm-based storage pool for the metavg. It
+will be possible to rename storage pools (thus also the default lvm storage
+pool). There will be new functionality to ask about what storage pools are
+available and of what type. Storage pools will have a storage pool type which is
+one of the disk templates. There can be more than one storage pool based on the
+same disk template, therefore we will then start referencing the storage pool
+name instead of the disk template.
+
+``gnt-cluster info`` will report which disk templates are enabled, i.e.
 which ones are supported according to the cluster configuration. Example
 output::
 
@@ -113,25 +223,26 @@ output::
   [...]
   Cluster parameters:
     - [...]
-    - enabled storage types: plain (default), drbd, lvm, rados
+    - enabled disk templates: plain, drbd, sharedfile, rados
     - [...]
 
 ``gnt-node list-storage`` will not be affected by any changes, since this design
-describes only free storage reporting for non-shared storage types.
+is restricted only to free storage reporting for non-shared storage types.
 
 Allocator changes
 -----------------
 
 The iallocator protocol doesn't need to change: since we know which
-storage type an instance has, we'll pass only the "free" value for that
-storage type to the iallocator, when asking for an allocation to be
+disk template an instance has, we'll pass only the "free" value for that
+disk template to the iallocator, when asking for an allocation to be
 made. Note that for DRBD nowadays we ignore the case when vg and metavg
-are different, and we only consider the main VG. Fixing this is outside
-the scope of this design.
+are different, and we only consider the main volume group. Fixing this is
+outside the scope of this design.
 
 With this design, we ensure forward-compatibility with respect to storage
-pools. For now, we'll report space for all available (non-shared) storage
-types, in the future, for all available storage pools.
+pools. For now, we'll report space for all available disk templates that
+are based on non-shared storage types, in the future, for all available
+storage pools.
 
 Rebalancing changes
 -------------------
@@ -143,7 +254,7 @@ Space reporting changes
 -----------------------
 
 Hspace will by default report by assuming the allocation will happen on
-the default storage for the cluster/nodegroup. An option will be added
+the default disk template for the cluster/nodegroup. An option will be added
 to manually specify a different storage.
 
 Interactions with Partitioned Ganeti
@@ -152,14 +263,15 @@ Interactions with Partitioned Ganeti
 Also the design for :doc:`Partitioned Ganeti <design-partitioned>` deals
 with reporting free space. Partitioned Ganeti has a different way to
 report free space for LVM on nodes where the ``exclusive_storage`` flag
-is set. That doesn't interact directly with this design, as the specific
+is set. That doesn't interact directly with this design, as the specifics
 of how the free space is computed is not in the scope of this design.
 But the ``node info`` call contains the value of the
 ``exclusive_storage`` flag, which is currently only meaningful for the
-LVM back-end. Additional flags like the ``exclusive_storage`` flag
-for lvm might be useful for other storage types as well. We therefore
-extend the RPC call with <type>,<key> to <type>,<key>,<params> to
-include any storage-type specific parameters in the RPC call.
+LVM storage type. Additional flags like the ``exclusive_storage`` flag
+for lvm might be useful for other disk templates / storage types as well.
+We therefore extend the RPC call with <disktemplate>,<key> to
+<disktemplate>,<key>,<params> to include any disk-template-specific
+(or storage-type specific) parameters in the RPC call.
 
 The reporting of free spindles, also part of Partitioned Ganeti, is not
 concerned with this design doc, as those are seen as a separate resource.