IBM Midrange System DS4000 Series Hardware Manual page 141

In this model, a read I/O request is handled where a host issues a read I/O request that is
sent over a path (such as a Fibre Channel) to the disk system. The request is received by a
disk system host adapter, which checks whether the requested data is already in cache. In
such a case, it is immediately sent back to the host. If the data is not in cache, the request is
forwarded to a disk adapter that reads the data from the appropriate disk and copies the data
into cache. The host adapter sends the data from cache to the requesting host.
Most (hardware) RAID controllers have some form of read or write caching. These caching
capabilities can be used, as they enhance the effective I/O capacity of the disk server. The
principle of these controller-based caching mechanisms is to gather smaller and potentially
nonsequential I/O requests coming in from the host server (for example, SQL Server) and try
to batch them with other I/O requests. Consequently, the I/O requests are sent as larger
(32 KB to 128 KB), and possibly sequential, requests to the hard disk drives. The RAID
controller cache arranges incoming I/O requests by making the best use of the hard disk's
underlying I/O processing ability. This increases the disk I/O throughput.
There are many different settings (related to caching) that come into play. The IBM System
Storage DS Storage Manager utility enables the following settings to be configured:
DS5000 system wide settings:
– Start and stop cache flushing levels (this setting will affect all arrays and logical drives
created on the system)
– Cache Block size
Logical drive specific settings:
– Read caching
– Dynamic read cache prefetch
– Write caching or write-through mode (write caching disabled)
– Enable or disable write cache mirroring
The default parameters configured in the Storage Manager are generally good for normal
installations. Select the defaults, or see 4.10.6, "Cache parameters" on page 249 for specific
configuration details.
When implementing a DS5000 storage subsystem as part of a whole solution, performance
testing and monitoring to adjust the settings should be planned over at least a one week
period.
4.1.7 Security planning:
Businesses must comply with a growing number of corporate standards and government
regulations, Drive Security is one tool that can enhance security by complying with these new
standards and regulations.
Full Disk Encryption
This is a new premium feature where Full Disk Encryption (FDE) protects the data on the
disks only when the drives are removed from storage subsystem enclosures. Drive Security
and FDE requires security capable drives and provide access to data only through a
controller that has the correct security key.
FDE does not prevent someone from copying the data in the storage subsystems through
Fibre Channel host port connections when the drives are unlocked and operating. FDE also
does not prevent unauthorized management access. A security capable drive encrypts data
during writes and decrypts data during reads.
Full Disk Encryption
Chapter 4. IBM System Storage DS planning and configuration
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