Learn About Enhanced Remote Mirroring

The is used for online, real-time data replication between over a remote distance. Storage subsystem manage the mirroring, which is transparent to machines and software applications. You create one or more mirrored logical drive pairs that consist of a at the primary site and a at a secondary, remote site. After you create the mirror relationship between the two , the of the primary logical drive copies all of the data from the primary logical drive to the secondary logical drive. This process is called a .


Attention:

Possible loss of data access – You cannot create a mirror relationship if the primary logical drive contains unreadable sectors. Furthermore, if an unreadable sector is discovered during a mirroring operation, the mirror relationship fails.


Note:

Because replication is managed on a per-logical drive basis, you can mirror individual logical drives in a primary storage subsystem to appropriate secondary logical drives in several different remote storage subsystems.

Disaster Recovery

The secondary, remote logical drive is unavailable to secondary host applications while mirroring is in progress. In the event of a disaster at the primary site, you can fail over to the secondary site. To fail over, perform a to promote the secondary logical drive to a primary logical drive. Then the recovery host is able to access the newly promoted logical drive, and business operations can continue.

Data Replication

When the current owner of the primary logical drive receives a write request from a host, the controller first logs information about the write to a special logical drive. This logical drive is called a . It writes the data to the primary logical drive. Next, the controller initiates a operation to copy the affected data blocks to the secondary logical drive at the remote site.

Finally, the controller sends an I/O completion indication back to the host system to confirm that the data was copied successfully to the secondary storage subsystem. The that you selected when you first created a determines when the I/O completion indication is sent to the host system.

The provides two write modes:

When is enabled on either the primary logical drive or the secondary logical drive, the I/O completion is sent when data is in the cache on the side (primary or secondary) where write caching is enabled. When write caching is disabled on either the primary logical drive or the secondary logical drive, the I/O completion is not sent until the data has been stored to physical media on that side.

Host write requests received by the controller are handled normally. No communication takes place between the primary storage subsystem and the secondary storage subsystem.

Link Interruptions or Secondary Logical Drive Errors

When processing write requests, the primary controller might be able to write to the primary logical drive, but a link interruption prevents communication with the remote secondary controller.

In this case, the remote write cannot complete to the secondary logical drive. The primary logical drive and the secondary logical drive are no longer appropriately mirrored. The primary controller changes the mirrored pair into and sends an I/O completion to the primary host. The primary host can continue to write to the primary logical drive, but remote writes do not take place.

When connectivity is restored between the current owner of the primary logical drive and the current owner of the secondary logical drive, a takes place. Only the blocks of data that have changed on the primary logical drive during the link interruption are copied to the secondary logical drive. The mirrored pair changes from an Unsynchronized state to .

The primary controller also marks the mirrored pair as Unsynchronized when a logical drive error on the secondary side prevents the remote write from completing. For example, an offline secondary logical drive or a failed secondary logical drive can cause the remote mirror to become unsynchronized. When the logical drive error is corrected (the secondary logical drive is placed online or is recovered to ), a full synchronization automatically begins. The mirrored pair then changes to Synchronization in Progress status.

Connectivity and Logical Drive Ownership

A primary controller attempts to communicate only with its matching controller in the secondary storage subsystem. For example, controller A in the primary storage subsystem attempts communication only with controller A in the secondary storage subsystem. The controller (A or B) that owns the primary logical drive determines the current owner of the secondary logical drive. If the primary logical drive is owned by controller A on the primary side, the secondary logical drive is owned by controller A on the secondary side. If primary controller A cannot communicate with secondary controller A, controller ownership changes do not take place.

The next remote write processed automatically triggers a matching ownership change on the secondary side if one of these conditions exists:

For example, a primary logical drive is owned by controller A, and then you change the controller owner to controller B. In this case, the next remote write changes the controller owner of the secondary logical drive from controller A to controller B. Because controller ownership changes on the secondary side are controlled by the primary side, they do not require any special intervention by the storage administrator.

Controller Resets and Storage Subsystem Power Cycles

Sometimes a remote write is interrupted by a controller reset or a storage subsystem power cycle before it can be written to the secondary logical drive. The storage subsystem controller does not need to perform a full synchronization of the mirrored logical drive pair in this case. A controller reset causes a controller ownership change on the primary side from the to the alternate controller in the storage subsystem. When a remote write has been interrupted during a controller reset, the new controller owner on the primary side reads information stored in a log file in the mirror repository logical drive of the preferred controller owner. It then copies the affected data blocks from the primary logical drive to the secondary logical drive, eliminating the need for a full synchronization of the mirrored logical drives.

Enhanced Remote Mirroring Premium Feature Activation

Like other premium features, you enable the Enhanced Remote Mirroring premium feature by purchasing a from your storage supplier. You must enable the premium feature on both the primary storage subsystem and the secondary storage subsystem.

Unlike other premium features, you also must activate the premium feature after you enable it. To activate the premium feature, use the Activate Enhanced Remote Mirroring Wizard in the Subsystem Management Window (SMW). Each controller in the storage subsystem must have its own mirror repository logical drive for logging write information to recover from controller resets and other temporary interruptions. The Activate Enhanced Remote Mirroring Wizard guides you to specify the placement of the two mirror repository logical drives (on newly created free capacity or existing free capacity in the storage subsystem).

After you activate the premium feature, one host side I/O on each controller is solely dedicated to Enhanced Remote Mirroring operations. Host-initiated I/O operations are not accepted by the dedicated port. I/O requests received on this port are accepted only from remote controllers that are participating in Enhanced Remote Mirroring operations with the controller.

Connectivity Requirements

You must attach dedicated Enhanced Remote Mirroring ports to a Fibre Channel fabric environment. In addition, these ports must support the Directory Service interface and the Name Service.

You can use a fabric configuration that is dedicated solely to the Enhanced Remote Mirroring ports on each controller. In this case, host systems can connect to the storage subsystems using fabric, , or configurations. These configurations are totally independent of the dedicated Enhanced Remote Mirroring fabric.

Alternatively, you can use a single Fibre Channel fabric configuration for both the Enhanced Remote Mirroring connectivity and for the host I/O paths to the controllers.

The maximum distance between the primary site and the secondary site is 10 km (6.2 miles), using single-mode fiber gigabit interface converters (GBICs) and optical long-wave GBICs.

Restrictions

These restrictions apply to mirrored logical drive candidates and storage subsystem mirroring: