If the FlashCopy® T-VOL is a TCz P-VOL, the FlashCopy® to PPRC Primary Volume function copies data from the FlashCopy® T-VOL to the TCz S-VOL. During the data copy, the status of the TCz pair is Pending. If a failure occurs, the consistency of the data on the TCz S-VOL cannot be guaranteed. The Preserve Mirror FlashCopy® function enables you to copy the FlashCopy® data to the secondary system with the status of the TCz pairs as Duplex.
The two options for establishing a FlashCopy® relationship using the Preserve Mirror FlashCopy® function are as follows:
- REQUIRED: Perform the copy operation only when the FlashCopy® relationship can be established using the Preserve Mirror FlashCopy® function. If the relationship cannot be established, the Preserve Mirror FlashCopy® command is rejected.
- PREFERRED: Perform the copy operation using the FlashCopy® to PPRC Primary Volume function, even if the FlashCopy® relationship cannot be established using the Preserve Mirror FlashCopy® function. In this case, the status of the TCz pair changes to Pending.
When using the Preserve Mirror FlashCopy® function, note the following:
- TCz is required.
- The status of the two TCz pairs must be Duplex. If they are not, the outcome of the operation depends on the specified (PRESMIRREQ or PRESMIRRPREF) option.
- Best practice is to set the MIH timer to 45 seconds when working with TCz synchronous pairs.
- If you create TCz pairs or resynchronize suspended pairs when FlashCopy® relationships exist, FlashCopy® prepares as many copies as the number of relationships. The larger the number of relationships per volume, the longer it takes for TCz pairs to become Duplex. Even with less load, it takes approximately 1.5 times longer when the TCz pairs are not used with FlashCopy®.
When using the Preserve Mirror FlashCopy® function, the maximum number of relationships (or datasets) per volume should be 2,000 or less. If that is not possible, you should create and operate one relationship for each volume. This causes non-data-set parts to be copied as well, but the status of the TCz pairs will change to Duplex in a shorter time.
- When you establish a FlashCopy® relationship with Preserve Mirror in COPY mode, a relationship might still be established in both the MCU and RCU. The copy operations for the relationship in the MCU and the relationship in the RCU are not synchronized.
The status of a TCz volume can be determined by the PPRC CQUERY command or through the CCI raidcom get remote_copy_pair command. To release this status, delete the TCz pair.
If the relationship at the primary site cannot be established, the relationship at the secondary site could be established, depending on the type of error. If you try again to create a FlashCopy® relationship using the Preserve Mirror FlashCopy® function, but the relationship will not be established at the primary site, release the relationship at the secondary site, and then try again.
When using the Preserve Mirror FlashCopy® function, the missing interrupt handler (MIH) value should be the same as the value for TCz operations. The best practice is to specify 60 seconds for the MIH value for TCz and 30 seconds for the RIO MIH time used by TCz path monitoring. For information about the RIO MIH time, see the TrueCopy® for Mainframe User Guide.
The zHyperWrite function and the Preserve Mirror FlashCopy® function can be used together. The following restrictions apply when using the zHyperWrite function with the Preserve Mirror FlashCopy® function:
- If the Preserve Mirror FlashCopy® function is used to back up the log volume being updated with zHyperWrite, there might be a difference in the backup data between the two FlashCopy® target volumes. This is expected behavior as an IBM® zHyperWrite program.
If there is a difference in the backup data between the target volumes, the storage system data can be recovered by reaquiring the backup data using the Preserve Mirror FlashCopy® function when there is no update by zHyperWrite.
- If a FlashCopy® relationship is established, the writing with zHyperWrite to the target TCz overwrites the FlashCopy® data. In addition, if the FlashCopy® relationship is established while the writing with zHyperWrite is in progress, the zHyperWrite data is overwritten by the copy operation with FlashCopy®.
The volume copy operation (of which a T-VOL is a TCz P-VOL) might not be performed by using some applications on the host. For details, see Why you use IBM Db2® Cloning Tool for z/OS®.
If a storage system failure occurs while an IBM OS utility or application such as DEFRAG is running the volume copy operation by using Preserve Mirror FlashCopy®, the data consistency between the P-VOL and S-VOL of a TCz pair might not be guaranteed even if the operation is complete normally and the TCz pair is in the Duplex status. For details, see Recovering if a storage system failure occurs during the volume copy processing with Preserve Mirror FlashCopy®.