ShadowImage for Mainframe (SIz) enables you to maintain storage system-internal copies of data volumes on the VSP One Block storage system for purposes such as data backup or duplication. The secondary volumes (S-VOLs) are created within the same storage system as the primary volumes (P‑VOLs) at hardware speeds.
When you create a ShadowImage for Mainframe pair, the storage system performs an initial copy operation to copy all of the data on the P-VOL to the S-VOL. During the initial copy operation, the P-VOL remains available for read and write operations from the host. After the initial copy is complete and the volumes are paired, the storage system uses update copy operations to keep the P-VOL and S-VOL paired.
The P-VOL and S-VOL remain paired until the pair is split. When you split a pair, the P-VOL continues to be updated by host I/O, but the updates in the P-VOL are not transferred to the S-VOL. The S-VOL contains a mirror image of the P-VOL at the time the pair was split.
- The S-VOL data is consistent and usable, available for read/write access by secondary host applications.
- Changes to the P-VOL and the S-VOL are managed by differential bitmaps.
- You can resynchronize the pair by updating the data from the P-VOL to the S-VOL, forward resynchronization, or from the S-VOL to the P-VOL, reverse resynchronization.
Initial copy operation
Unpaired volumes are called simplex volumes (SMPL status). When you create a pair and the initial copy operation starts, the pair status changes to CPPD. When the initial copy operation completes, the P-VOL and S-VOL are paired and have the PAIR status.
The following figure illustrates the ShadowImage for Mainframe initial copy operation.
Update copy operation
After the initial copy operation is complete, the P-VOL and S-VOL are paired and the status changes to PAIR. The storage system keeps track of updates to the P-VOL and periodically performs update copy operations to transfer the P-VOL updates to the S-VOL. When the P-VOL is updated, the storage system marks the I/O to the P-VOL as differential data and stores the location of the data in bitmaps for asynchronous transfer to the S-VOL. The timing of update copy operations is based on the amount of differential data that has accumulated and the elapsed time since the previous update copy operation was performed.
The following figure illustrates the ShadowImage for Mainframe update copy operation.