FlashCopy® is often used to make copies of data that frequently crosses the volume boundary. In such cases, the data that crosses the volume boundary needs to be consistent, that is, it must be copied in the proper order. FlashCopy Consistency groups, available in the System Data Mover API, provide a mechanism for achieving a consistent data copy across multiple volumes without requiring that application I/O be quiesced.
In the case of production data, application impact must be minimized. Without consistency groups, you must first quiesce the application, establish the FlashCopy relationships, and then restart the application. This process can be very disruptive, causing application outages or data unavailability for an unacceptable period of time.
For PPRC or XRC, the process of creating consistent point-in-time copies at a secondary site can be equally disruptive. And, during the process, data is not being copied to the secondary site. In general, the process consists of the following tasks:
The FlashCopy consistency group function improves this process for data-consistent copies or dependent write-consistent copies using both PPRC and XRC. Consistency group formation is initiated when you specify FREEZE in the ACTION parameter with a FlashCopy Establish request. When this request is received, the ESS gives a long busy to the specified volumes, stalling write I/O completion to the FlashCopy source. Once the Establish request has been issued for all participating FlashCopy relationships, issue a FlashCopy Withdraw request specifying THAW in the ACTION parameter to each LSS containing participating volumes so that write I/O can resume. Write I/O automatically resumes after 120 seconds even if the FlashCopy Withdraw THAW request is not issued.
The freeze operation affects the entire volume. Even if the FlashCopy Establish request contains extent specifications, the entire volume will remain busy to host operations, but the FlashCopy relationship between the source and target will be established as specified using whatever parameters were issued with the request.
Since the request to form a consistency group is processed by the LSS for all volumes in the LSS, multiple consistency groups with volumes in the same LSS must not be formed at the same time.
If a FlashCopy Withdraw request with THAW specified is received by an LSS that does not have any volumes in a consistency group being formed, the command is accepted, but no actual processing takes place.
Since the command to end a consistency group is processed at the LSS level, the target device information specified via the ANTRQST API or via the TSO command only needs to contain valid hexadecimal values for parsing purposes.