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# SRDF session modes

SRDF transparently remotely mirrors production or primary (source) site data to a secondary (target) site to users, applications, databases, and host processors.

Mode | Description
--- | ---
Adaptive Copy | This mode allows the source (R1) volume and target (R2) volume to be out of synchronization by a number of I/O operations per second that are defined by a skew value.
Adaptive copy disk mode | Data is read from the disk, and the unit of transfer across the SRDF link is the entire track. While less global memory is consumed, it is typically slower to read data from disk than from global memory. Also, more bandwidth is used because the unit of transfer is the entire track. Also, because it is slower to read data from disk than global memory, device resynchronization time increases.
Adaptive Copy WP Mode | The unit of transfer across the SRDF link is the updated blocks rather than an entire track, resulting in more efficient use of SRDF link bandwidth. Data is read from global memory instead of disk, thus improving overall system performance. However, the global memory is temporarily consumed by the data until it is transferred across the link.
Synchronous | This mode 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 devices. 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 device can be confirmed when the current SRDF/A cycle commits the data to disk by successfully de-staging it to the R2 storage volumes.
AC Skew | Adaptive Copy Skew - sets the number of tracks per volume the source volume can be ahead of the target volume. Values are 0–65535.