568 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
10.7.9 Global Mirror Overview
This section describes the Global Mirror copy service, which is an asynchronous remote copy
service. This service provides and maintains a consistent mirrored copy of a source volume to
a target volume.
Global Mirror establishes a Global Mirror relationship between two volumes of equal size. The
volumes in a Global Mirror relationship are referred to as the
master (source) volume and the
auxiliary (target) volume, which is the same as Metro Mirror. Consistency Groups can be
used to maintain data integrity for dependent writes, which is similar to FlashCopy
Consistency Groups.
Global Mirror writes data to the auxiliary volume asynchronously, which means that host
writes to the master volume provide the host with confirmation that the write is complete
before the I/O completes on the auxiliary volume.
Global Mirror has the following characteristics:
Near-zero RPO
Asynchronous
Production application performance that is affected by I/O sequencing preparation time
Intracluster Global Mirror
Although Global Mirror is available for intracluster, it has no functional value for production
use. Intracluster Metro Mirror provides the same capability with less processor use. However,
leaving this functionality in place simplifies testing and supports client experimentation and
testing (for example, to validate server failover on a single test system). As with Intracluster
Metro Mirror, you must consider the increase in the license requirement, because source and
target exist on the same IBM Spectrum Virtualize system.
Intercluster Global Mirror
Intercluster Global Mirror operations require a pair of IBM Spectrum Virtualize systems that
are connected by several intercluster links. The two systems must be defined in a partnership
to establish a fully functional Global Mirror relationship.
10.7.10 Asynchronous remote copy
Global Mirror is an asynchronous remote copy technique. In asynchronous remote copy, the
write operations are completed on the primary site and the write acknowledgment is sent to
the host before it is received at the secondary site. An update of this write operation is sent to
the secondary site at a later stage, which provides the capability to perform remote copy over
distances that exceed the limitations of synchronous remote copy.
The Global Mirror function provides the same function as Metro Mirror remote copy, but over
long-distance links with higher latency without requiring the hosts to wait for the full round-trip
delay of the long-distance link.
Figure 10-81 on page 569 shows that a write operation to the master volume is
acknowledged back to the host that is issuing the write before the write operation is mirrored
to the cache for the auxiliary volume.
Limit: When a local fabric and a remote fabric are connected for Global Mirror purposes,
the ISL hop count between a local node and a remote node must not exceed seven hops.
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