564 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
Nodes that are in separate systems do not exchange messages after initial discovery is
complete, unless they are configured together to perform a remote copy relationship.
The intercluster link carries control traffic to coordinate activity between two systems. The link
is formed between one node in each system. The traffic between the designated nodes is
distributed among logins that exist between those nodes.
If the designated node fails (or all of its logins to the remote system fail), a new node is
chosen to carry control traffic. This node change causes the I/O to pause, but it does not put
the relationships in a ConsistentStopped state.
10.7.4 Metro Mirror overview
Metro Mirror establishes a synchronous relationship between two volumes of equal size. The
volumes in a Metro Mirror relationship are referred to as the master (primary) volume and the
auxiliary (secondary) volume. Traditional FC Metro Mirror is primarily used in a metropolitan
area or geographical area, up to a maximum distance of 300 km (186.4 miles) to provide
synchronous replication of data.
With synchronous copies, host applications write to the master volume, but they do not
receive confirmation that the write operation completed until the data is written to the auxiliary
volume. This action ensures that both the volumes have identical data when the copy
completes. After the initial copy completes, the Metro Mirror function always maintains a fully
synchronized copy of the source data at the target site.
Metro Mirror has the following characteristics:
Zero recovery point objective (RPO)
Synchronous
Production application performance that is affected by round-trip latency
Increased distance directly affects host I/O performance because the writes are synchronous.
Use the requirements for application performance when you are selecting your Metro Mirror
auxiliary location.
Consistency Groups can be used to maintain data integrity for dependent writes, which is
similar to FlashCopy Consistency Groups (FlashCopy Consistency Groups are described in
10.3, “Implementing FlashCopy” on page 492).
The IBM Spectrum Virtualize provides intracluster and intercluster Metro Mirror.
Intracluster Metro Mirror
Intracluster Metro Mirror performs the intracluster copying of a volume, in which both volumes
belong to the same system and I/O Group within the system. Because it is within the same
I/O Group, there must be sufficient bitmap space within the I/O Group for both sets of volumes
and licensing on the system.
Note: It is advised to use chsystem with -partnerfcportmask to dedicate several FC ports
only to system-to-system traffic to ensure that remote copy is not affected by other traffic,
such as host-to-node traffic or node-to-node traffic within the same system.
Important: Performing Metro Mirror across I/O Groups within a system is not supported.
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