Practical Use Of Metro Mirror - IBM Storwize V5010 Manual

To access the auxiliary volume, the Metro Mirror relationship must be stopped with the
access option enabled, before write I/O is allowed to the auxiliary.
The remote host server is mapped to the auxiliary volume, and the disk is available for I/O.

10.7.8 Practical use of Metro Mirror

The master volume is the production volume, and updates to this copy are mirrored in real
time to the auxiliary volume. The contents of the auxiliary volume that existed when the
relationship was created are deleted.
Switching copy direction: The copy direction for a Metro Mirror relationship can be
switched so that the auxiliary volume becomes the master, and the master volume
becomes the auxiliary, which is similar to the FlashCopy restore option. However, although
the FlashCopy target volume can operate in read/write mode, the target volume of the
started remote copy is always in read-only mode.
While the Metro Mirror relationship is active, the auxiliary volume is not accessible for host
application write I/O at any time. The IBM Storwize V5000 Gen2 system allows read-only
access to the auxiliary volume when it contains a consistent image. IBM Storwize allows boot
time operating system discovery to complete without an error, so that any hosts at the
secondary site can be ready to start the applications with minimum delay, if required.
For example, many operating systems must read LBA zero to configure a logical unit.
Although read access is allowed at the auxiliary in practice, the data on the auxiliary volumes
cannot be read by a host because most operating systems write a "dirty bit" to the file system
when it is mounted. Because this write operation is not allowed on the auxiliary volume, the
volume cannot be mounted.
This access is provided only where consistency can be ensured. However, coherency cannot
be maintained between reads that are performed at the auxiliary and later write I/Os that are
performed at the master.
To enable access to the auxiliary volume for host operations, you must stop the Metro Mirror
relationship by specifying the -access parameter. While access to the auxiliary volume for
host operations is enabled, the host must be instructed to mount the volume before the
application can be started, or instructed to perform a recovery process.
For example, the Metro Mirror requirement to enable the auxiliary copy for access
differentiates it from third-party mirroring software on the host, which aims to emulate a
single, reliable disk regardless of what system is accessing it. Metro Mirror retains the
property that there are two volumes in existence but it suppresses one volume while the copy
is being maintained.
The use of an auxiliary copy demands a conscious policy decision by the administrator that a
failover is required, and that the tasks to be performed on the host that is involved in
establishing the operation on the auxiliary copy are substantial. The goal is to make this copy
rapid (much faster when compared to recovering from a backup copy) but not seamless.
The failover process can be automated through failover management software. The IBM
Storwize V5000 Gen2 system provides SNMP traps and programming (or scripting) for the
CLI to enable this automation.
Chapter 10. Copy services
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