When a PPRC multi-target configuration is created, two TCz secondary sites are paired. The pair is called an MTIR pair. PPRC multi-target configuration using MTIR enables you to continue operation in two secondary sites with data duplicated when a disaster occurs on the primary site. After the primary site is recovered, the operation with two secondary sites can be changed to the original PPRC multi-target configuration quickly.
An MTIR pair is created between the secondary sites after the second TCz pair is created from the primary site. When the MTIR pair is created, the pair status is Suspend and is shown as Primary in both secondary sites.
When operations are changed from the primary site to the secondary site, data is copied between the secondary sites (MTIR pair) by Failback operation. This copy is called an incremental resync.
The copy operation depends on whether a pair supports the differential data copy function in a PPRC multi-target configuration. You can use the CCI raidcom get remote_copy_pair command to check whether a pair supports the differential data copy function in a PPRC multi-target configuration.
| Pair type | Copy operation |
|---|---|
| Pair not supporting differential data copying in PPRC multi-target configuration (pair is created on VSP One B85 with DKCMAIN version earlier than A0-05-41) | Full data copy |
| Pair supporting differential data copying in PPRC multi-target configuration (pair is created on VSP One B85 or VSP 5000 series with DKCMAIN version A0-05-41 or later) | Differential data copy |
| Pair consisting of a volume from a pair supporting and a volume from a pair not supporting differential data copying in PPRC multi-target configuration | Full data copy |
To switch the copy operation from a full data copy to a differential data copy, update the microcode at all sites to a version that supports the differential data copy function in the PPRC multi-target configuration, and then suspend and resynchronize the TCz pair.
The copy pace during the incremental resync operation might be slower than an initial copy because incremental resync acts the same as a pair resync operation. The following figure shows a PPRC multi-target configuration using MTIR.
The differential data copy function in the PPRC multi-target configuration uses a differential bitmap dedicated to this function to manage differences in track units. When data updated by writing to the primary site from the host is transferred to the secondary site, the corresponding differential bit is enabled. The differential bit is asynchronously disabled by the internal processing of the storage system. Immediately after completing pair creation or pair resynchronization in the PPRC multi-target configuration, the entire area of the differential bitmap is enabled.
The difference status can be verified by using the PPRCCQUERY command. If there are differences, the synchronization rate of the MTIR pair is displayed in the range of 0% through 99%. While writing from the host to the primary site continues, the synchronization rate of the MTIR pair transitions between 0% and 99%. If the data between the secondary sites match and disabling the differential bit is complete, the synchronization rate of the MTIR pair will be 100%. However, the synchronization rate of the MTIR pair might not reach 100% even after waiting for a certain period of time due to a hardware failure or the like. If this is the case, the synchronization rate of the MTIR pair can be recalculated by performing pair split and pair resynchronization. The incremental resync operation can be performed even if the synchronization rate of the MTIR pair is in the range of 0% through 99%.
When an MTIR pair is automatically created, use CCI or PPRC commands to set the following options:
- Primary Volume Fence Level
- CCI: Never
- PPRC CQUERY command: PPRC CRIT=NO
- CFW Data
- CCI: Use the raidcom add remote_copy_pair -cfw_svol command to set.
- DEVSERVE: CFW:Y
- DFW to Secondary Volume
- CCI: Use the raidcom add remote_copy_pair -cfw_svol command to set.
- DEVSERVE: DFW:YY
You can change the default option values by selecting the MTIR pair and manually changing the values. The options that are not listed above are not set automatically. During the incremental resync operation, the specified option is enabled.