manila/share_replication.rst at b780ef9ef36e7e4bce3fdba8895e445f8d7eb88b

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As of the Mitaka release of OpenStack, manila supports replication of shares between different pools for drivers that operate with driver_handles_share_servers=False mode. These pools may be on different backends or within the same backend. This feature can be used as a disaster recovery solution or as a load sharing mirroring solution depending upon the replication style chosen, the capability of the driver and the configuration of backends.

This feature assumes and relies on the fact that share drivers will be responsible for communicating with ALL storage controllers necessary to achieve any replication tasks, even if that involves sending commands to other storage controllers in other Availability Zones (or AZs).

End users would be able to create and manage their replicas, alongside their shares and snapshots.

Storage availability zones and replication domains

Replication is supported within the same availability zone, but in an ideal solution, an Availability Zone should be perceived as a single failure domain. So this feature provides the most value in an inter-AZ replication use case.

The replication_domain option is a backend specific StrOpt option to be used within manila.conf. The value can be any ASCII string. Two backends that can replicate between each other would have the same replication_domain. This comes from the premise that manila expects Share Replication to be performed between backends that have similar characteristics.

When scheduling new replicas, the scheduler takes into account the replication_domain option to match similar backends. It also ensures that only one replica can be scheduled per pool. When backends report multiple pools, manila would allow for replication between two pools on the same backend.

The replication_domain option is meant to be used in conjunction with the storage_availability_zone (or back end specific backend_availability_zone) option to utilize this solution for Data Protection/Disaster Recovery.

Replication types

When creating a share that is meant to have replicas in the future, the user will use a share_type with an extra_spec, replication_type set to a valid replication type that manila supports. Drivers must report the replication type that they support as the replication_type capability during the _update_share_stats() call.

Three types of replication are currently supported:

writable: Synchronously replicated shares where all replicas are writable. Promotion is not supported and not needed.
readable: Mirror-style replication with a primary (writable) copy and one or more secondary (read-only) copies which can become writable after a promotion.
dr (for Disaster Recovery): Generalized replication with secondary copies that are inaccessible until they are promoted to become the active replica.

Note

The term active replica refers to the primary share. In writable style of replication, all replicas are active, and there could be no distinction of a primary share. In readable and dr styles of replication, a secondary replica may be referred to as passive, non-active or simply replica.

Apart from the status attribute, share replicas have the replica_state attribute to denote the state of the replica. The primary replica will have it's replica_state attribute set to active. A secondary replica may have one of the following values as its replica_state:

in_sync: The replica is up to date with the active replica (possibly within a backend specific recovery point objective).
out_of_sync: The replica has gone out of date (all new replicas start out in this replica_state).
error: When the scheduler failed to schedule this replica or some potentially irrecoverable damage occurred with regard to updating data for this replica.

Manila requests periodic update of the replica_state of all non-active replicas. The update occurs with respect to an interval defined through the replica_state_update_interval option in manila.conf.

Administrators have an option of initiating a resync of a secondary replica (for readable and dr types of replication). This could be performed before a planned failover operation in order to have the most up-to-date data on the replica.

Promotion

For readable and dr styles, we refer to the task of switching a non-active replica with the active replica as promotion. For the writable style of replication, promotion does not make sense since all replicas are active (or writable) at all given points of time.

The status attribute of the non-active replica being promoted will be set to replication_change during its promotion. This has been classified as a busy state and hence API interactions with the share are restricted while one of its replicas is in this state.

Promotion of replicas with replica_state set to error may not be fully supported by the backend. However, manila allows the action as an administrator feature and such an attempt may be honored by backends if possible.

When multiple replicas exist, multiple replication relationships between shares may need to be redefined at the backend during the promotion operation. If the driver fails at this stage, the replicas may be left in an inconsistent state. The share manager will set all replicas to have the status attribute set to error. Recovery from this state would require administrator intervention.

Snapshots

If the driver supports snapshots, the replication of a snapshot is expected to be initiated simultaneously with the creation of the snapshot on the active replica. Manila tracks snapshots across replicas as separate snapshot instances. The aggregate snapshot object itself will be in creating state until it is available across all of the share's replicas that have their replica_state attribute set to active or in_sync.

Therefore, for a driver that supports snapshots, the definition of being in_sync with the primary is not only that data is ensured (within the recovery point objective), but also that any 'available' snapshots on the primary are ensured on the replica as well. If the snapshots cannot be ensured, the replica_state must be reported to manila as being out_of_sync until the snapshots have been replicated.

When a snapshot instance has its status attribute set to creating or deleting, manila will poll the respective drivers for a status update. As described earlier, the parent snapshot itself will be available only when its instances across the active and in_sync replicas of the share are available. The polling interval will be the same as replica_state_update_interval.

Access Rules

Access rules are not meant to be different across the replicas of the share. Manila expects drivers to handle these access rules effectively depending on the style of replication supported. For example, the dr style of replication does mean that the non-active replicas are inaccessible, so if read-write rules are expected, then the rules should be applied on the active replica only. Similarly, drivers that support readable replication type should apply any read-write rules as read-only for the non-active replicas.

Drivers will receive all the access rules in create_replica, delete_replica and update_replica_state calls and have ample opportunity to reconcile these rules effectively across replicas.

Understanding Replication Workflows

Administrators can create a share type with extra-spec replication_type, matching the style of replication the desired backend supports. Users can use the share type to create a new share that allows/supports replication. A replicated share always starts out with one replica, the primary share itself.

The manila-scheduler service will filter and weigh available pools to find a suitable pool for the share being created. In particular,

The CapabilityFilter will match the replication_type extra_spec in the request share_type with the replication_type capability reported by a pool.
The ShareReplicationFilter will further ensure that the pool has a non-empty replication_domain capability being reported as well.
The AvailabilityZoneFilter will ensure that the availability_zone requested matches with the pool's availability zone.

Creating a replica

The user has to specify the share name/id of the share that is supposed to be replicated and optionally an availability zone for the replica to exist in. The replica inherits the parent share's share_type and associated extra_specs. Scheduling of the replica is similar to that of the share.

The ShareReplicationFilter will ensure that the pool is within

the same replication_domain as the active replica and also ensures that the pool does not already have a replica for that share.

Drivers supporting writable style must set the replica_state attribute to active when the replica has been created and is available.

Deleting a replica

Users can remove replicas that have their status attribute set to error, in_sync or out_of_sync. They could even delete an active replica as long as there is another active replica (as could be the case with writable replication style). Before the delete_replica call is made to the driver, an update_access call is made to ensure access rules are safely removed for the replica.

Administrators may also force-delete replicas. Any driver exceptions will only be logged and not re-raised; the replica will be purged from manila's database.

Promoting a replica

Users can promote replicas that have their replica_state attribute set to in_sync. Administrators can attempt to promote replicas that have their replica_state attribute set to out_of_sync or error. During a promotion, if the driver raises an exception, all replicas will have their status attribute set to error and recovery from this state will require administrator intervention.

Resyncing a replica

Prior to a planned failover, an administrator could attempt to update the data on the replica. The update_replica_state call will be made during such an action, giving drivers an opportunity to push the latest updates from the active replica to the secondaries.

Creating a snapshot

When a user takes a snapshot of a share that has replicas, manila creates as many snapshot instances as there are share replicas. These snapshot instances all begin with their status attribute set to creating. The driver is expected to create the snapshot of the active replica and then begin to replicate this snapshot as soon as the active replica's snapshot instance is created and becomes available.

Deleting a snapshot

When a user deletes a snapshot, the snapshot instances corresponding to each replica of the share have their status attribute set to deleting. Drivers must update their secondaries as soon as the active replica's snapshot instance is deleted.

Driver Interfaces

As part of the _update_share_stats() call, the base driver reports the replication_domain capability. Drivers are expected to update the replication_type capability.

Drivers must implement the methods enumerated below in order to support replication. promote_replica, update_replica_state and update_replicated_snapshot need not be implemented by drivers that support the writable style of replication. The snapshot methods create_replicated_snapshot, delete_replicated_snapshot and update_replicated_snapshot need not be implemented by a driver that does not support snapshots.

Each driver request is made on a specific host. Create/delete operations on secondary replicas are always made on the destination host. Create/delete operations on snapshots are always made on the active replica's host. update_replica_state and update_replicated_snapshot calls are made on the host that the replica or snapshot resides on.

manila.share.driver.ShareDriver

Replicated Snapshot interfaces: