Theory of Operation: Mapping NetApp to OpenStack Terminology

Theory of Operation: Mapping NetApp to OpenStack Terminology

Cinder Backends and Storage Virtual Machines

Storage Virtual Machines (SVMs, formerly known as Vservers) contain data volumes and one or more LIFs through which they serve data to clients. SVMs can either contain one or more FlexVol volumes.

SVMs securely isolate the shared virtualized data storage and network, and each SVM appears as a single dedicated storage virtual machine to clients. Each SVM has a separate administrator authentication domain and can be managed independently by its SVM administrator.

In a cluster, SVMs facilitate data access. A cluster must have at least one SVM to serve data. SVMs use the storage and network resources of the cluster. However, the volumes and LIFs are exclusive to the SVM. Multiple SVMs can coexist in a single cluster without being bound to any node in a cluster. However, they are bound to the physical cluster on which they exist.


When deploying Cinder with ONTAP, NetApp recommends that each Cinder backend refer to a single SVM within a cluster through the use of the netapp_vserver configuration option. While the driver can operate without the explicit declaration of a mapping between a backend and an SVM, a variety of advanced functionality (e.g. volume type extra-specs) will be disabled.

Cinder volumes and FlexVol Volumes

ONTAP FlexVol volumes (commonly referred to as volumes) and OpenStack Block Storage volumes (commonly referred to as Cinder volumes) are not semantically analogous. A FlexVol volume is a container of logical data elements (for example: files, Snapshot copies, clones, LUNs, etc.) that is abstracted from physical elements (for example: individual disks, and RAID groups). A Cinder volume is a block device. Most commonly, these block devices are made available to OpenStack Compute instances. NetApp’s various driver options for deployment of FAS and All Flash FAS (AFF) as a provider of Cinder storage place Cinder volumes, snapshot copies, and clones within FlexVol volumes.


The FlexVol volume is an overarching container for one or more Cinder volumes.


NetApp’s OpenStack Cinder drivers are not supported for use with Infinite Volumes, as ONTAP currently only supports FlexClone files and FlexClone LUNs with FlexVol volumes.

Cinder volume Representation within a FlexVol Volume

A Cinder volume has a different representation in ONTAP when stored in a FlexVol volume, dependent on storage protocol utilized with Cinder:

  • iSCSI: When utilizing the iSCSI storage protocol, a Cinder volume is stored as an iSCSI LUN.

  • NFS: When utilizing the NFS storage protocol, a Cinder volume is a file on an NFS export.

  • Fibre Channel: When utilizing the Fibre Channel storage protocol, a Cinder volume is stored as a Fibre Channel LUN.

Cinder Scheduling and Resource Pool Selection

When Cinder volumes are created, the Cinder scheduler selects a resource pool from the available storage pools: see Storage Pools for an overview. Table 4.10, “Behavioral Differences in Cinder volume Placement” details the behavioral changes in NetApp’s Cinder drivers when scheduling the provisioning of new Cinder volumes.

Beginning with Juno, each of NetApp’s Cinder drivers report per-pool capacity to the scheduler. When a new volume is provisioned, the scheduler capacity filter eliminates too-small pools from consideration. Similarly, the scheduler’s capability, availability zone and other filters narrow down the list of potential backends that may receive a new volume based on the volume type and other volume characteristics.

The scheduler also has an evaluator filter that evaluates an optional arithmetic expression to determine whether a pool may contain a new volume. Beginning with Mitaka, the ONTAP drivers report per-pool controller utilization values to the scheduler, along with a “filter function” that prevents new volumes from being created on pools that are overutilized. Controller utilization is computed by the drivers as a function of CPU utilization and other internal I/O metrics. The default filter function supplied by the ONTAP drivers is “capabilities.utilization < 70”. A utilization of 70% is a good starting point beyond which I/O throughput and latency may be adversely affected by additional Cinder volumes. The filter function may be overridden on a per-backend basis in the Cinder configuration file. See Configure and use driver filter and weighing for scheduler for details about using the evaluator functions in Cinder.

Each candidate pool that passes the filters is then considered by the scheduler’s weighers so that the optimum one is chosen for a new volume. As of Juno, the Cinder scheduler has per-pool capacity information, and the scheduler capacity weigher may be configured to spread new volumes among backends uniformly or to fill one backend before using another.

Beginning with Mitaka, the ONTAP drivers report per-pool controller utilization values to the scheduler, along with a “goodness function” that allows the scheduler to prioritize backends that are less utilized. Controller utilization is reported as a percentage, and the goodness function is expected to yield a value between 0 and 100, with 100 representing maximum “goodness”. The default goodness function supplied by the ONTAP drivers is “100 - capabilities.utilization”, and it may be overridden on a per-backend basis in the Cinder configuration file.


The storage controller utilization metrics are reported by the Mitaka Cinder drivers for ONTAP 8.2 or higher.

Beginning with Newton, additional information such as aggregate name and aggregate space utilization is reported to the scheduler and may be used in filter and weigher expressions. For example, to keep from filling an aggregate completely, a filter expression of “capabilities.aggregate_used_percent < 80” might be used.

Beginning with Ocata, additional information such as per-pool consumption of the ONTAP shared block limit is reported to the scheduler and may be used in filter and weigher expressions. For example, to steer new Cinder volumes away from a FlexVol nearing its shared block limit, a filter expression of “capabilities.netapp_dedupe_used_percent < 90” might be used.


Scheduling Behavior (as of Juno)

Scheduling Behavior (as of Mitaka)


Each FlexVol volume’s capacity and SSC data is reported separately as a pool to the Cinder scheduler. The Cinder filters and weighers decide which pool a new volume goes into, and the driver honors that request.

Same as Juno. Also, per-pool storage controller utilization is reported to the scheduler, along with filter and goodness expressions that take controller utilization into account when making placement decisions.

Table 4.10. Behavioral Differences in Cinder volume Placement

Cinder Snapshots versus NetApp Snapshots

A NetApp Snapshot copy is a point-in-time file system image. Low-overhead NetApp Snapshot copies are made possible by the unique features of the WAFL storage virtualization technology that is part of ONTAP. The high performance of the NetApp Snapshot makes it highly scalable. A NetApp Snapshot takes only a few seconds to create — typically less than one second, regardless of the size of the volume or the level of activity on the NetApp storage system. After a Snapshot copy has been created, changes to data objects are reflected in updates to the current version of the objects, as if NetApp Snapshot copies did not exist. Meanwhile, the NetApp Snapshot version of the data remains completely stable. A NetApp Snapshot incurs no performance overhead; users can comfortably store up to 255 NetApp Snapshot copies per FlexVol volume, all of which are accessible as read-only and online versions of the data.

Since NetApp Snapshots are taken at the FlexVol level, they can not be directly leveraged within an OpenStack context, as a user of Cinder requests a snapshot be taken of a particular Cinder volume (not the containing FlexVol volume). As a Cinder volume is represented as either a file on NFS or as a LUN (in the case of iSCSI or Fibre Channel), the way that Cinder snapshots are created is through use of ONTAP’s FlexClone technology. By leveraging the FlexClone technology to facilitate Cinder snapshots, it is possible to create many thousands of Cinder snapshots for a single Cinder volume.

FlexClone files or FlexClone LUNs and their parent files or LUNs that are present in the FlexClone volume continue to share blocks the same way they do in the parent FlexVol volume. In fact, all the FlexClone entities and their parents share the same underlying physical data blocks, minimizing physical disk space usage.


When Cinder is deployed with ONTAP, Cinder snapshots are created leveraging the FlexClone feature of ONTAP. As such, a license option for FlexClone must be enabled.

ONTAP Consistency Groups

ONTAP currently has “Consistency Group” snapshot operations, but their semantics are not identical to Cinder CG operations. Cinder CGs are tenant-defined sets of Cinder-volumes that act together as a unit for a snapshot. ONTAP currently has no actual “Consistency Group” object, but only CG snapshot operations. Moreover, these operations act on ONTAP volumes, flexvols, which are themselves containers of the backing files or LUNs for Cinder volumes. In effect, so long as there is room in a Cinder pool to fit a snapshot or a copy of a consistency group, that operation will be permitted without any further restriction.