Metro Mirror and Global Mirror

Metro Mirror and Global Mirror are two types of remote-copy operations. You can use these functions to set up a relationship between two volumes, where updates made to one volume are mirrored on the other volume. For Metro Mirror and Global Mirror operations, the volumes can be on two different systems (intersystem). Metro Mirror and Global Mirror operations also support volumes that reside on the same system (intrasystem). To use Metro Mirror and Global Mirror functions, the remote-copy license must be installed on each enclosure that you plan to use these functions.

Although data is only written to a single volume, the system maintains two copies of the data. If the copies are separated by a significant distance, the Metro Mirror and Global Mirror copies can be used as a backup for disaster recovery. A prerequisite for Metro Mirror and Global Mirror operations between systems over Fibre Channel connections is that the SAN fabric to which they are attached provides adequate bandwidth between the systems. SAN fabrics are not required for IP-only connections.

For both Metro Mirror and Global Mirror copy types, one volume is designated as the primary and the other volume is designated as the secondary. Host applications write data to the primary volume, and updates to the primary volume are copied to the secondary volume. Normally, host applications do not run I/O operations to the secondary volume.

Metro Mirror is a type of remote copy that creates a synchronous copy of data from a primary volume to a secondary volume. A secondary volume can either be on the same system or on another system. With synchronous copies, host applications write to the primary volume but do not receive confirmation that the write operation has completed until the data is written to the secondary volume. This ensures that both the volumes have identical data when the copy operation completes. After the initial copy operation completes, the Metro Mirror function maintains a fully synchronized copy of the source data at the target site at all times.

To use the Global Mirror function, all components in the network must be capable of sustaining the workload that is generated by application hosts and the Global Mirror background copy process. If all of the components in the network cannot sustain the workload, the Global Mirror relationships are automatically stopped to protect your application hosts from increased response times.

When Global Mirror operates without cycling, write operations are applied to the secondary volume as soon as possible after they are applied to the primary volume. The secondary volume is generally less than 1 second behind the primary volume, which minimizes the amount of data that must be recovered if a failover occurs. However, a high-bandwidth link must be provisioned between the two sites.

The system supports the following types of relationships:
Active-active
This type of relationship is only created for HyperSwap volumes. When HyperSwap is configured on the system, the HyperSwap volumes are located on separate sites and an active-active relationship is automatically configured between them. Updates to the volumes in the relationship are updated simultaneously on both sites to provide disaster recovery solutions for the system.
Active-active
This type of relationship is only created for HyperSwap volumes. When HyperSwap is configured on the system, the HyperSwap volumes are located on separate sites and an active-active relationship is automatically configured between them. Updates to the volumes in the relationship are updated simultaneously on both sites to provide disaster recovery solutions for the system.
Note: Some systems do not support HyperSwap configurations. Verify support of these configurations before using this function.
Metro Mirror

Metro Mirror is a type of remote copy that creates a synchronous copy of data from a primary volume to a secondary volume. A secondary volume can either be on the same system or on another system.

With synchronous copies, host applications write to the primary volume but do not receive confirmation that the write operation has completed until the data is written to the secondary volume. This ensures that both the volumes have identical data when the copy operation completes. After the initial copy operation completes, the Metro Mirror function maintains a fully synchronized copy of the source data at the target site at all times.

The Metro Mirror function supports copy operations between volumes that are separated by distances up to 300 km. For disaster recovery purposes, Metro Mirror provides the simplest way to maintain an identical copy on both the primary and secondary volumes. However, like with all synchronous copies over remote distances, there can be a performance impact to host applications. This performance impact is related to the distance between primary and secondary volumes and depending on application requirements, its use might be limited based on the distance between sites.

Global Mirror without cycling (cycling mode set to None)

The Global Mirror function provides an asynchronous copy process. When a host writes to the primary volume, confirmation of I/O completion is received before the write operation completes for the copy on the secondary volume.

If a failover operation is initiated, the application must recover and apply any updates that were not committed to the secondary volume. If I/O operations on the primary volume are paused for a small length of time, the secondary volume can become an exact match of the primary volume. This function is comparable to a continuous backup process in which the last few updates are always missing. When you use Global Mirror for disaster recovery, you must consider how you want to handle these missing updates.

To use the Global Mirror function, all components in the network must be capable of sustaining the workload that is generated by application hosts and the Global Mirror background copy process. If all of the components in the network cannot sustain the workload, the Global Mirror relationships are automatically stopped to protect your application hosts from increased response times.

When Global Mirror operates without cycling, write operations are applied to the secondary volume as soon as possible after they are applied to the primary volume. The secondary volume is generally less than 1 second behind the primary volume, which minimizes the amount of data that must be recovered if a failover occurs. However, a high-bandwidth link must be provisioned between the two sites.

Global Mirror with change volumes (cycling mode set to Multiple)
Global Mirror with change volumes (cycling mode set to Multiple) provides the same basic function of asynchronous copy operations between source and target volumes for disaster recovery.

If you are using Global Mirror with cycling mode set to Multiple, the copying process is similar to Metro Mirror and standard Global Mirror. Change volumes must be configured for both the primary and secondary volumes in each relationship. A copy is taken of the primary volume in the relationship using the change volume that is specified when the Global Mirror relationship with change volumes is created. The background copy process reads data from the stable and consistent change volume, copying the data to the secondary volume in the relationship. Copy-on-write technology is used to maintain the consistent image of the primary volume for the background copy process to read. The changes that took place while the background copy process was active are also tracked. The change volume for the secondary volume can also be used to maintain a consistent image of the secondary volume while the background copy process is active.

Supported functions and operations

The Metro Mirror and Global Mirror functions support the following operations:
  • Intersystem copying of a volume, in which one volume belongs to a system and the other volume belongs to a different system.
    Note: A system can participate in active Metro Mirror and Global Mirror relationships with up to three other systems.
  • Intrasystem copying of a volume, in which both volumes belong to the same system.
    Note: Intrasystem copying is not supported for Global Mirror relationships.
  • Intersystem and intrasystem relationships can be used concurrently on the same system.
  • Bidirectional links are supported for intersystem relationships. This means that data can be copied from system A to system B for one pair of volumes while data is copied from system B to system A for a different pair of volumes.
  • The copy direction can be reversed for a consistent relationship.
  • You can change the copy type for relationships and consistency groups between Metro Mirror and Global Mirror with or without change volumes.
  • Consistency groups are supported to manage a group of relationships that must be kept synchronized for the same application. This also simplifies administration because a command that is issued to the consistency group is applied to all the relationships in that group.
  • The maximum number of remote-copy relationships varies per system. Verify the maximum number of remote copy relationships for your model.Lenovo Storage V3700 V2 systems support up to 2048 relationships. Lenovo Storage V3700 V2 XP systems support up to 4096 remote-copy relationships. Lenovo Storage V5030 and Lenovo Storage V5030F systems support up to 8192 remote-copy relationships.

Changing the copy type of relationships and consistency groups

As part of system administration, you can change the copy type of relationships and consistency groups between Metro Mirror and Global Mirror. To do so, you can use the management GUI or CLI commands. For more information, see Changing the copy type of remote-copy relationships and consistency groups.