COPY-MODE (processing mode)

Remote copying is available in synchronous, adaptive-copy or asynchronous processing mode. The initial settings are made when the remote copy pair is created. The processing mode can be changed using the COPY-MODE operand of /MODIFY-REMOTE-COPY-PARAMETER.

• SYNCHronous processing mode

  Processing mode = synchronous processing. All writes are written via the server channel to the cache of the local storage system and are forwarded from there to the cache of the remote storage system. The local system is notified on successful completion. Only then the next write can be performed.

  With this processing mode, data is written on the source unit and the target unit after every successful write.Synchronous processing mode is the most secure mode.

  When synchronous processing mode is set using /MODIFY-REMOTE-COPY-PARAMETER WAIT=*UNTIL-SYNCHRONIZATION, the system waits until the remote copy pair is synchronized again. This is possible only when the remote copy status is ACTIVE. In dialog mode you can terminate waiting by hitting the K2 key.

  
  

• ADAPTIVE-COPY processing mode

  Processing mode = asynchronous processing. In this processing mode there may be more than one pending write.
  The maximum value for pending writes can be set explicitly using the PENDING-UPDATE-LIMIT operand in /MODIFY-REMOTE-COPY-PARAMETER. If the number of pending writes exceeds the set limit, synchronous processing mode is set automatically until the number of pending writes falls below the limit again.

  This processing mode is available in two submodes which use different strategies for noting pending write. You can use /MODIFY-REMOTE-COPY-PARAMETER and the WRITE-BUFFER=*DISK or WRITE-BUFFER=*CACHE operand to specify whether pending writes are noted on the source unit or in the cache of the local storage system.

  The processing mode ADAPTIVE-COPY(WRITE-BUFFER=*CACHE) is not supported if the source unit resides on VMAX3. See the commands /MODIFY-REMOTE-COPY-PARAMTER on "MODIFY-REMOTE-COPY-PARAMETER Changes the processing mode and remote copy parameters" and /START-REMOTE-COPY on "START-REMOTE-COPY Creates a remote copy pair".

  In ADAPTIVE-COPY processing mode, performance improves during write load, but remote copy reliability is reduced because the source and target units may differ by several writes.

  If ADAPTIVE-COPY processing mode is terminated, all pending writes are transferred before a new write is forwarded from the server. If the unprotected source unit fails (i.e. no RAID protection) when WRITE-BUFFER=*DISK is set, all data not yet transferred is lost.

  The WRITE-BUFFER=*DISK setting should be used for data migration. The WRITE-BUFFER=*CACHE setting should be used if there is a high write load with reproducible writes.
  Because the data still to be transferred is buffered in the cache, the cache space available for other actions is reduced. That may negatively impact throughput.

  
  

• ASYNCHronous processing mode (SRDF/A)

  SRDF/A is an asynchronous replication function that features cross-volume data consistency. SRDF/A is fully integrated into SRDF and employs the technology and infrastructure of SRDF. Starting with another common copy mode of SRDF (generally “synchronous” or “adaptive copy” mode), an “SRDF/A session” is initiated in which “asynchronous” copy mode is set. The SRDF/A session is terminated by setting a different SRDF copy mode.

  In this processing mode there may be more than one pending write. The data on the target units of an SRDF/A session is however always in a state that is consistent across volumes even in the event of failures because the writes are transmitted in bundled form and in the correct order.

  Constraints for SRDF/A:

  • Operation and control only for all devices of the RA group at the same time

  • The entry UNIT=*BY-PUBSET is possible if the volumes of the pubset match the volumes of the RA group exactly.