Key Concepts¶

Volume¶

A Cinder volume is the fundamental resource unit allocated by the Block Storage service. It represents an allocation of persistent, readable, and writable block storage that could be utilized as the root disk for a compute instance, or as secondary storage that could be attached and/or detached from a compute instance. The underlying connection between the consumer of the volume and the Cinder service providing the volume can be achieved with the iSCSI, NFS, or Fibre Channel storage protocols (dependent on the support of the Cinder driver deployed).

Cinder volumes can be identified uniquely through a UUID assigned by the Cinder service at the time of volume creation. A Cinder volume may also be optionally referred to by a human-readable name, though this string is not guaranteed to be unique within a single tenant or deployment of Cinder.

The actual blocks provisioned in support of a Cinder volume reside on a single Cinder backend. Starting in the Havana release, a Cinder volume can be migrated from one storage backend to another within a deployment of the Cinder service; refer to the section called “Cinder Command Line Interface (CLI)” for an example of volume migration.

The cinder manage command allows importing existing storage objects that are not managed by Cinder into new Cinder volumes. The operation will attempt to locate an object within a specified Cinder backend and create the necessary metadata within the Cinder database to allow it to be managed like any other Cinder volume. The operation will also rename the volume to a name appropriate to the particular Cinder driver in use. The imported storage object could be a file, LUN, or a volume depending on the protocol (iSCSI/FC/NFS). This feature is useful in migration scenarios where virtual machines or other data need to be managed by Cinder; refer to the section called “Cinder Manage Usage” for an example of the cinder manage command.

The cinder unmanage command allows Cinder to cease management of a particular Cinder volume. All data stored in the Cinder database related to the volume is removed, but the volume’s backing file, LUN, or appropriate storage object is not deleted. This allows the volume to be transferred to another environment for other use cases; refer to the section called “Cinder Unmanage Usage” for an example of the cinder unmanage command.

Snapshot¶

A Cinder snapshot is a point-in-time, read-only copy of a Cinder volume. Snapshots can be created from an existing Cinder volume that is operational and either attached to an instance or in a detached state. A Cinder snapshot can serve as the content source for a new Cinder volume when the Cinder volume is created with the create from snapshot option specified or can be used to revert a volume to the most recent snapshot using the revert to snapshot feature (as of the Pike release).

Backend¶

A Cinder backend is the configuration object that represents a single provider of block storage upon which provisioning requests may be fulfilled. A Cinder backend communicates with the storage system through a Cinder driver. Cinder supports multiple backends to be simultaneously configured and managed (even with the same Cinder driver) as of the Grizzly release.

Driver¶

A Cinder driver is a particular implementation of a Cinder backend that maps the abstract APIs and primitives of Cinder to appropriate constructs within the particular storage solution underpinning the Cinder backend.

Volume Type¶

A Cinder volume type is an abstract collection of criteria used to characterize Cinder volumes. They are most commonly used to create a hierarchy of functional capabilities that represent a tiered level of storage services; for example, a cloud administrator might define a premium volume type that indicates a greater level of performance than a basic volume type, which would represent a best-effort level of performance.

The collection of criteria is specified as a list of key/value pairs, which are inspected by the Cinder scheduler when determining which Cinder backend(s) are able to fulfill a provisioning request. Individual Cinder drivers (and subsequently Cinder backends) may advertise arbitrary key/value pairs (also referred to as capabilities) to the Cinder scheduler, which are then compared against volume type definitions when determining which backend will fulfill a provisioning request.

Extra Spec¶

An extra spec is a key/value pair, expressed in the style of key=value. Extra specs are associated with Cinder volume types, so that when users request volumes of a particular volume type, the volumes are created on storage backends that meet the specified criteria.

Quality of Service¶

The Cinder Quality of Service (QoS) support for volumes can be enforced either at the hypervisor or at the storage subsystem (backend), or both.

SolidFire

Within the SolidFire platform, each volume may be configured with minimum, maximum, and burst IOPS values that are strictly enforced within the system. The minimum IOPS provides a guarantee for performance, independent of what other applications on the system are doing. The maximum and burst values control the allocation of performance and deliver consistent performance to workloads.

QoS support for the SolidFire drivers includes the ability to set the following capabilities in the OpenStack Block Storage API cinder.api.contrib.qos_specs_manage qos specs extension module:

Table 4.1a. SolidFire QoS Options

ONTAP

The NetApp ONTAP Cinder driver currently supports QoS by backend QoS specs or via netapp:qos_policy_group assignment using Cinder Extra-Specs. The NetApp Cinder driver accomplishes this by using NetApp QoS policy groups, introduced with ONTAP 8.2, and applying these policy groups to Cinder volumes.

• netapp:qos_policy_group: A Cinder extra-spec, which references an externally provisioned QoS policy group, provides a means to assign a Netapp QoS policy group for a set of Cinder volumes. All Cinder volumes associated with a single QoS policy group share the throughput value restrictions as a group. The ONTAP QoS policy group must be created by the storage administrator on the backend prior to specifying the netapp:qos_policy_group option in a Cinder extra-spec. Use the netapp:qos_policy_group option when a Service Level Objective (SLO) needs to be applied to a set of Cinder volumes. For more information on this, see Table 4.11, “NetApp supported Extra Specs for use with Cinder volume Types”.

• QoS Spec: QoS specifications are added as standalone objects that can then be associated with Cinder volume types. A Cinder QoS Spec will create a new NetApp QoS policy group for each Cinder volume. A Cinder QoS spec can specify the maximum bytes per second or IOPS throughput value. This throughput value is the maximum for a single Cinder volume created with this QoS spec. When deleting a Cinder volume that has a QoS Spec applied, the NetApp QoS policies associated with that Cinder volume will not immediately be deleted. The driver marks the QoS policies for deletion by the NetApp QoS policy reaping job. The NetApp QoS policy reaping job runs every 60 seconds. Refer to NetApp ONTAP documentation for your version of ONTAP to determine NetApp QoS policy group limits. Use the QoS Spec feature when a SLO needs to be applied to a single Cinder volume.

Table 4.1b. NetApp Supported Backend QoS Spec Options

Storage Pools¶

With the Juno release of OpenStack, Cinder has introduced the concept of “storage pools”. The backend storage may present one or more logical storage resource pools from which Cinder will select as a storage location when provisioning volumes. In releases prior to Juno, NetApp’s Cinder drivers contained some logic that determined which FlexVol volume, volume group, or DDP a Cinder volume would be placed into; with the introduction of pools, all scheduling logic is performed completely within the Cinder scheduler.

For NetApp’s Cinder drivers, a Cinder pool is a single container. The container that is mapped to a Cinder pool is dependent on the storage protocol used:

• iSCSI and Fibre Channel: a Cinder pool is created for every FlexVol volume within the SVM specified by the configuration option netapp_vserver, or for ONTAP, all FlexVol volumes within the system unless limited by the configuration option netapp_pool_name_search_pattern.

• NFS: a Cinder pool is created for each junction path from FlexVol volumes that are listed in the configuration option nfs_shares_config.

For additional information, refer to Cinder Scheduling and Resource Pool Selection.

Generic Volume Groups¶

With the Newton release of OpenStack, NetApp supports Generic Volume Groups when ONTAP iSCSI/Fibre Channel and ONTAP NFS drivers. For SolidFire, Generic Volume Groups support is provided since the Pike release. Existing consistency group operations will be migrated to use generic volume group operations in future releases. The existing Consistency Group construct cannot be extended easily to serve purposes such as consistent group snapshots across multiple volumes. A generic volume group makes it possible to group volumes used in the same application and these volumes do not have to support consistent group snapshot. It provides a means of managing volumes and grouping them based on a common factor. Additional information about volume groups and the proposed migration can be found at generic-volume-groups

Consistency Groups¶

With the Mitaka release of OpenStack, NetApp supports Cinder Consistency Groups when using ONTAP iSCSI/Fibre Channel drivers. With the Newton release of OpenStack, NetApp supports Cinder Consistency Groups when using ONTAP NFS drivers. Consistency group support allows snapshots of multiple volumes in the same consistency group to be taken at the same point-in-time to ensure data consistency. To illustrate the usefulness of consistency groups, consider a bank account database where a transaction log is written to Cinder volume V1 and the account table itself is written to Cinder volume V2. Suppose that $100 is to be transferred from account A to account B via the following sequence of writes:

1. Log start of transaction.

2. Log remove $100 from account A.

3. Log add $100 to account B.

4. Log commit transaction.

5. Update table A to reflect -$100.

6. Update table B to reflect +$100.

Writes 1-4 go to Cinder volume V1 whereas writes 5-6 go to Cinder volume V2. To see that we need to keep write order fidelity in both snapshots of V1 and V2, suppose a snapshot is in progress during writes 1-6, and suppose that the snapshot completes at a point where writes 1-3 and 5 have completed, but not 4 and 6. Because write 4 (log of commit transaction) did not complete, the transaction will be discarded. But write 5 has completed anyways, so a restore from snapshot of the secondary will result in a corrupt account database, one where account A has been debited $100 without account B getting the corresponding credit.

Before using consistency groups, you must change policies for the consistency group APIs in the /etc/cinder/policy.json file. By default, the consistency group APIs are disabled. Enable them before running consistency group operations. Here are existing policy entries for consistency groups:

"consistencygroup:create": "group:nobody",
"consistencygroup:delete": "group:nobody",
"consistencygroup:update": "group:nobody",
"consistencygroup:get": "group:nobody",
"consistencygroup:get_all": "group:nobody",
"consistencygroup:create_cgsnapshot" : "group:nobody",
"consistencygroup:delete_cgsnapshot": "group:nobody",
"consistencygroup:get_cgsnapshot": "group:nobody",
"consistencygroup:get_all_cgsnapshots": "group:nobody",

Remove group:nobody to enable these APIs:

"consistencygroup:create": "",
"consistencygroup:delete": "",
"consistencygroup:update": "",
"consistencygroup:get": "",
"consistencygroup:get_all": "",
"consistencygroup:create_cgsnapshot" : "",
"consistencygroup:delete_cgsnapshot": "",
"consistencygroup:get_cgsnapshot": "",
"consistencygroup:get_all_cgsnapshots": "",

Remember to restart the Block Storage API service after changing policies.

The NetApp Driver creates consistency group LUN snapshots thick provisioned. This can be changed on the backend after the snap is taken with no effect to Cinder.

Backup and Restore¶

Cinder offers OpenStack tenants self-service backup and restore operations for their Cinder volumes. These operations are performed on individual volumes. A Cinder backup operation creates a point-in-time, read-only set of data and metadata that can be used to restore the contents of a single Cinder volume either to a new Cinder volume (the default) or to an existing Cinder volume. In contrast to snapshots, backups are stored in a dedicated repository, independent of the storage pool containing the original volume or the storage backend providing its block storage.

Cinder backup repositories may be implemented either using an object store (such as Swift) or by using an NFS shared filesystem. The Cinder backup service uses a single repository, irrespective of the backends used to provide storage pools for the volumes themselves. For example, a FlexVol volume exported from an ONTAP storage system using NFS can serve as a backup repository for multi-backend, heterogeneous Cinder deployments.

Tenant-controlled, per-volume backup service is complementary to, but not a replacement for, administrative backups of the storage pools themselves that hold Cinder volumes. See http://netapp.github.io/openstack/2015/03/12/cinder-backup-restore/ for a valuable approach to administrative backups when ONTAP storage pools are used to host Cinder volumes.

Disaster Recovery¶

In the Newton release of OpenStack, NetApp’s Cinder driver for ONTAP (for FC, NFS, iSCSI) was updated to match Cinder’s v2.1 spec for replication. This makes it possible to replicate an entire backend, and allow all replicated volumes across different pools to fail over together. Intended to be a disaster recovery mechanism, it provides a way to configure one or more disaster recovery partner storage systems for your Cinder backend. For more details on the configuration and failover process, refer to Cinder Replication with NetApp

Revert to Snapshot¶

As of Pike release, Cinder supports revert to snapshot feature. This feature can be used to overwrite the current state and data of a volume to the most recent snapshot taken. The volume can not be reverted if it was extended after taking the snapshot.

An optimized implementation of the revert to snapshot operation is used for the SolidFire driver. For ONTAP backends, the feature works by using a generic implementation that works for NFS/iSCSI/FC driver modes.