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).
Warning
A Cinder volume is an abstract storage object that may or may not directly map to a “volume” concept from the underlying backend provider of storage. It is critically important to understand this distinction, particularly in context of a Cinder deployment that leverages NetApp storage solutions.
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.
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).
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.
Note
A single Cinder backend may be defined in the [DEFAULT] stanza
of cinder.conf; however, NetApp recommends that the
enabled_backends configuration option be set to a
comma-separated list of backend names, and each backend name have
its own configuration stanza with the same name as listed in the
enabled_backends option. Refer to the section called
“Cinder.conf: Overview” for an example of the use of this option.
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.
Caution
The use of the term “driver” often creates confusion given common understanding of the behavior of “device drivers” in operating systems. The term can connote software that provides a data I/O path. In the case of Cinder driver implementations, the software provides provisioning and other manipulation of storage devices but does not lay in the path of data I/O. For this reason, the term “driver” is often used interchangeably with the alternative (and perhaps more appropriate) term “provider”.
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.
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.
Note
The list of default capabilities that may be reported by a Cinder driver and included in a volume type definition include:
volume_backend_name: The name of the backend as defined in
cinder.confvendor_name: The name of the vendor who has implemented the
driver (e.g. NetApp)driver_version: The version of the driver (e.g. 1.0)storage_protocol: The protocol used by the backend to export
block storage to clients (e.g. iSCSI, fc, or nfs)For a table of NetApp supported extra specs, refer to Table 4.11, “NetApp supported Extra Specs for use with Cinder volume Types”.
Note
NetApp drivers support multi-attachment of volumes for NFS, iSCSI
and FC protocols from the Rocky release. This enables attaching a
volume to multiple servers simultaneously and can be configured
by creating an extra-spec multiattach=True for the associated
Cinder volume type.
Note
In-use volume extension is not supported by NetApp drivers. Currently, it is not possible to extend the size of a volume that is attached to a server and uses a NetApp backend. Extension can only be done by detaching the volume from the server.
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:
| Option | Description |
|---|---|
| minIOPS | The minimum number of IOPS guaranteed for this volume. Default = 100. |
| maxIOPS | The maximum number of IOPS allowed for this volume. Default = 15,000. |
| burstIOPS | The maximum number of IOPS allowed over a short period of time. Default = 15,000. |
Table 4.1a. SolidFire QoS Options
Note
The SolidFire driver utilizes volume-types for QoS settings and allows dynamic changes to QoS.
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.
| Option | Description |
|---|---|
| maxBPS | The maximum bytes per second allowed. |
| maxBPSperGiB | The maximum bytes per second allowed per GiB of Cinder volume capacity. |
| maxIOPS | The maximum IOPS allowed. |
| maxIOPSperGiB | The maximum IOPS allowed per GiB of Cinder volume capacity. |
Table 4.1b. NetApp Supported Backend QoS Spec Options
Warning
While SolidFire supports volume retyping, ONTAP does not.
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:
netapp_vserver, or for ONTAP, all
FlexVol volumes within the system unless limited by the configuration
option netapp_pool_name_search_pattern.nfs_shares_config.For additional information, refer to Cinder Scheduling and Resource Pool Selection.
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
Note
Only Block Storage V3 API supports groups. The minimum version for
group operations supported by the ONTAP drivers is 3.14. The API
version can be specified with the following CLI flag
--os-volume-api-version 3.14
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:
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.
Note
Consistency group operations support has been deprecated in Block Storage V3 API. Only Block Storage V2 API supports consistency groups. Future releases will involve a migration of existing consistency group operations to use generic volume group operations.
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.
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
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.
This document is licensed under Apache 2.0 license.