IBM N series Hardware Manual page 217

Enterprise applications
Direct-attached storage (DAS) architectures that are used to deploy enterprise applications
have significantly different requirements than the home directory environment. It is common
for the emphasis to be on performance, uptime, and backup rather than on flexibility and
individual file recovery.
Commonly, these environments use a block protocol, such as iSCSI or FCP because they
mimic DAS more closely than NAS technologies. However, increasingly the advantages and
flexibility that is provided by NAS solutions are drawing more attention. Rather than being
designed to serve individual files, the configuration focuses on LUNs or the use of files as
though they were LUNs. An example is a database application that uses files for its storage
instead of LUNs. At its most fundamental, the database application does not treat I/O to files
any differently than it does to LUNs. This configuration allows you to choose the deployment
that provides the combination of flexibility and performance required.
Enterprise environments often are deployed with their storage systems clustered. This
configuration minimizes the possibility of a service outage that is caused by a failure of the
storage appliance. In clustered environments, there is always the opportunity to spread
workload across at least two active storage systems. Therefore, getting good throughput for
the enterprise application is not difficult.
The application administrator should have a good understanding of the type of different
workloads so that beneficial balancing can be accomplished. Clustered environments always
have multiple I/O paths available, so it is important to balance the workload across these I/O
paths and across server heads.
For mission-critical environments, it is important to plan for the worst-case scenario. That is,
running the enterprise when one of the storage systems fails and the remaining single unit
must provide the entire load. In most circumstances, the mere fact that the enterprise is
running despite a significant failure is viewed as positive. However, there are situations in
which the full performance expectation must be met even after a failure. In this case, the
storage systems must be sized accordingly.
Block protocols with iSCSI or FCP are also common. The use of a few files or LUNs to
support the enterprise application means that the distribution of the workload is relatively
easy to install and predict.
Microsoft Exchange
Microsoft Exchange has various parameters that affect the total storage that is required of
N series. These parameters are shown in the following examples:
Number of instances
With Microsoft Exchange, you can specify how many instances of an email or document
are saved. The default is 1. If you elect to save multiple instances, take this into
consideration for storage sizing.
Number of logs kept
Microsoft Exchange uses a 5 MB log size. The data change rate determines the number of
logs that are generated per day for recovery purposes. A highly active Microsoft Exchange
server can generate up to 100 logs per day.
Number of users
This number, along with mailbox limit, user load, and percentage concurrent access, has a
significant effect on the sizing.
Chapter 14. Designing an N series solution
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