488 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
10.1.5 Application testing with FlashCopy
It is often important to test a new version of an application or operating system that is using
actual production data. This testing ensures the highest quality possible for your environment.
FlashCopy makes this type of testing easy to accomplish without putting the production data
at risk or requiring downtime to create a constant copy.
You create a FlashCopy of your source and use that for your testing. This copy is a duplicate
of your production data down to the block level so that even physical disk identifiers are
copied. Therefore, it is impossible for your applications to tell the difference.
10.1.6 Host and application considerations to ensure FlashCopy integrity
Because FlashCopy is at the block level, it is necessary to understand the interaction
between your application and the host operating system. From a logical standpoint, it is
easiest to think of these objects as “layers” that sit on top of one another. The application is
the topmost layer, and beneath it is the operating system layer.
Both of these layers have various levels and methods of caching data to provide better speed.
Because the FlashCopy sit below these layers, they are unaware of the cache at the
application or operating system layers.
To ensure the integrity of the copy that is made, it is necessary to flush the host operating
system and application cache for any outstanding reads or writes before the FlashCopy
operation is performed. Failing to flush the host operating system and application cache
produces what is referred to as a
crash consistent copy.
The resulting copy requires the same type of recovery procedure, such as log replay and file
system checks, that is required following a host crash. FlashCopies that are crash consistent
often can be used following file system and application recovery procedures.
Various operating systems and applications provide facilities to stop I/O operations and
ensure that all data is flushed from host cache. If these facilities are available, they can be
used to prepare for a FlashCopy operation. When this type of facility is unavailable, the host
cache must be flushed manually by quiescing the application and unmounting the file system
or drives.
10.1.7 FlashCopy attributes
The FlashCopy function in IBM Spectrum Virtualize features the following attributes:
The target is the time-zero copy of the source, which is known as
FlashCopy mapping
targets
.
FlashCopy produces an exact copy of the source volume, including any metadata that was
written by the host operating system, logical volume manager, and applications.
Preferred practice: From a practical standpoint, when you have an application that is
backed by a database and you want to make a FlashCopy of that application’s data, it is
sufficient in most cases to use the write-suspend method that is available in most modern
databases, because the database maintains strict control over I/O.
This method is as opposed to flushing data from both the application and the backing
database, which is always the suggested method because it is safer. However, this method
can be used when facilities do not exist or your environment includes time sensitivity.
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