IBM Midrange System DS4000 Series Hardware Manual page 130

Tip: Having more physical disks for the same overall capacity gives you:
Performance: By doubling the number of the physical drives, you can expect up to a
50% increase in transactional performance.
Flexibility: Using more physical drives gives you more flexibility to build arrays and
logical drives according to your needs.
Data capacity: When using RAID 5 logical drives, more data space is available with
smaller physical drives because less space (capacity of a drive) is used for parity.
However, pay attention to the protection level, because with larger disk drives, the
rebuild time might expose you to a second failure and lead to an offline array.
Enclosure layout and loss protection planning
Depending on the DS storage system and expansions in your configuration, you need to plan
in advance how these expansions will be best interconnected with your DS system controller.
In order to optimize performance from each controller to each expansion, you need to
determine the best layout of your cabling configuration.
Especially in large configurations with multiple expansion enclosures, you can provide better
controller access to the drives, and thus optimize performance, by selecting array members
from different enclosures. Other considerations to optimize the DS storage subsystem is to
evenly spread disk array members from odd and even slots, because each controller has a
preferred path to odd or even slots.
Enclosure loss protection is a good way to make your system more resilient against hardware
failures. Enclosure loss protection means that you spread your protection arrays across
multiple enclosures rather than in one enclosure so that a failure of a single enclosure does
not take a whole array offline.
By default, the automatic configuration is enabled. However, this is not a best practice for the
method of creating arrays. Instead, use the manual method, as this allows for more
configuration options to be available at creation time.
Best practice: Manual array configuration allows for greater control over the creation of
arrays because it allows you to specify your planned optimal configuration options.
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IBM Midrange System Storage Hardware Guide