SRDF/A configurations

SRDF/A is the asynchronous replication function for Symmetrix/VMAX3 that features cross-volume data consistency.

SRDF/A mirrors data over long distances either with either no impact or minimal impact on the application. Data consistency is guaranteed at all times.

SRDF/A is fully integrated into SRDF and employs the technology and infrastructure of SRDF. Starting with another copy mode of SRDF (generally “synchronous” or “adaptive copy” mode), an “SRDF/A session” is initiated in which “asynchronous” copy mode is set. This SRDF/A session is terminated by setting a different copy mode. In an SRDF/A session the local data is buffered in special cache areas and is sent cyclically to the remote storage system. The maximum delta between the local data and the consistent data status on the remote storage system is the amount of data modified locally during a single cycle. Default cycle duration is 30 seconds.

SRDF/A control and status

The initial state for SRDF/A is an existing SRDF replication. For reasons of data consistency, it is advisable to base SRDF/A on synchronous mode.

SRDF/A status

• ACTIVE
  SRDF replication takes place in asynchronous SRDF/A mode in the ACTIVE status.

• INACTIVE
  SRDF replication is suspended (IN-HOLD) or takes place in synchronous or adaptive-copy mode.

SRDF/A control

• Activating SRDF/A – this corresponds to switching to asynchronous copy mode. The SRFD/A status becomes ACTIVE.

  • The data on the target units is consistent immediately if synchronous copy mode was set before SRDF/A is activated.

  • Transmission of the invalid tracks starts immediately if adaptive-copy mode was set before SRDF/A is activated. The data on the target units is not consistent until transmission has been completed.

• Deactivating SRDF/A – this corresponds to switching to synchronous or adaptive-copy mode of SRDF. The SRDF/A session is terminated, SRDF replication continues, data consistency on the target side is possibly no longer guaranteed.

  • When synchronous copy mode is switched to, the CONSISTENT operand in the /MODIFY-REMOTE-COPY-PARAMETERS command controls whether or not the data is kept consistently for the duration of any synchronization which may be required (default value: the data is not kept consistently).

  • Consistency is lost if a switch is made to adaptive-copy mode.

• Terminating the SRDF/A session – this corresponds to direct transition from ACTIVE to INACTIVE.

  There are two ways of terminating the SRDF/A session, both of which guarantee data consistency of the SRDF pair:

  • Immediate termination (e.g. after failure of the SRDF links); there are then invalid tracks on the source and target side.

  • Termination after completion of the current cycle; there are then invalid tracks on the source side only, e.g. as a result of /HOLD-REMOTE-COPY.

  In the event of termination, the CONSISTENCY-PROTECT operand of the /MODIFY-REMOTE-COPY-PARAMETERS command controls the data consistency of all SRDF pairs in an RA group.

SRDF/A parameters

The duration of a cycle can be set by customer support. The minimum value and also the default value is 30 seconds.