Metro/Global copy mode combinations

Table 1 shows which mode combinations are supported in a cascading relationship.

Table 1. Supported Metro/Global Copy Mode Combinations
Local Volume Pair	Remote Volume Pair
Synchronous PPRC	Synchronous PPRC
Synchronous PPRC	PPRC Extended Distance
PPRC Extended Distance	PPRC Extended Distance

The combination of PPRC-XD to synchronous PPRC is not supported because the intermediate volume B, a fuzzy copy, cannot be the source for a consistent copy at the tertiary volume.

Synchronous PPRC to PPRC-XD is the typical configuration intended for disaster recovery. The combination of PPRC-XD to PPRC-XD allows you to maintain multiple copies of data without signification impact to application I/O performance. In contrast, a combination of two synchronous PPRC relationships will cause a suspend on the first I/O to the first PPRC primary in the cascaded triad. This configuration may be useful to guarantee a consistent copy at the tertiary volume if there is little write I/O and the whole distance exceeds the maximum of 103 km supported for a single synchronous PPRC.

While following the rule that prohibits the two volumes that form a PPRC pair from being in the same LSS, it is theoretically possible to have these logical subsystems in the same ESS, however it is only practical to cascade PPRC pairs two ways:

Three ESSs with one volume per ESS, as shown in Figure 1
When the local pair runs in synchronous mode, keep the distance between the local and intermediate ESSs as close as possible to minimize the performance impact. The intermediate ESS should be in a secure environment, separated from the local ESS, with separate power to reduce the possibility of an outage affecting both locations.

The remote ESS is typically in a difference city, any distance from the intermediate ESS. If the remote PPRC pair operates asynchronously from the local PPRC pair, the effect of long-distance on host response time is minimized.
Two ESSs with volumes A and B in the same local ESS and volume C in a remote ESS
If the remote pair operates in PPRC-XD mode, this configuration is similar to a single PPRC-XD relationship. The intermediate volume allows procedures for consistent, remote, point in time copy with less application impact.

Setting up Global Mirror for ESS describes how to set up Global Mirror.

Setting up a Metro/Global Mirror configuration describes how to set up Metro/Global Mirror.