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# SRDF group modes

SRDF groups provide a collective data transfer path linking volumes of two separate storage systems.

The following values can be set for SRDF groups:

Synchronous—This setting provides the host access to the source (R1) volume on a write operation only after the storage system containing the target (R2) volume acknowledges that it has received and checked the data.

Asynchronous—The storage system acknowledges all writes to the source (R1) volumes as if they were local volumes. Host writes accumulate on the source (R1) side until the cycle time is reached and are transferred to the target (R2) volume in one delta set. Write operations to the target volume can be confirmed when the current SRDF/A cycle commits the data to disk by successfully de-staging it to the R2 storage volumes.

Semi Synchronous—The storage system containing the source (R1) volume informs the host of successful completion of the write operation when it receives the data. The SRDF (RA) director transfers each write to the target (R2) volume as the SRDF links become available. The storage system containing the target (R2) volume checks and acknowledges receipt of each write.

AC WP Mode On—(adaptive copy write pending) the storage system acknowledges all writes to the source (R1) volume as if it was a local volume. The new data accumulates in the cache until it is successfully written to the source (R1) volume and the remote director has transferred the write to the target (R2) volume.

AC Disk Mode On—This setting is used for situations requiring the transfer of large amounts of data without loss of performance. Use this mode to temporarily transfer the bulk of your data to target (R2) volumes, then switch to synchronous or semi-synchronous mode.

Domino Mode On—Ensures that the data on the source (R1) and target (R2) volumes are always synchronized. The storage system forces the source (R1) volume to a Not Ready state to the host whenever it detects one side in a remotely mirrored pair is unavailable.

Domino Mode Off—The remotely mirrored volume continues processing I/O operations per second with its host, even when an SRDF volume or link failure occurs.

AC Mode Off—Turns off the AC disk mode.

AC Change Skew—Modifies the adaptive copy skew threshold. When the skew threshold is exceeded, the remotely mirrored pair operates in the predetermined SRDF state (synchronous or semi-synchronous). When the number of invalid tracks drops below this value, the remotely mirrored pair reverts to the adaptive copy mode.

(R2 NR If Invalid) On—Sets the R2 device to Not Ready when there are invalid tracks.

(R2 NR If Invalid) Of—Turns off the (R2 NR_If_Invalid) On mode.