IBM Midrange System DS4000 Series Hardware Manual page 52

Figure 3-8 shows a DS5300 controller without the memory board and Host Interface Cards.
Figure 3-8 DS5000 controller without memory board and Host Interface Cards
Data cache memory
The controller cache has a large amount of physical memory dedicated to I/O operations
between the controller and hosts and between the controller and disk drives. It is logically and
physically separate from the controller processor's memory and can participate in Direct
Memory Access (DMA) operations with both host side and drive side physical channel
adapters, as shown in Figure 3-9 on page 35. Consequently, the controller's processor is not
required to execute the data movement in and out of the cache memory. The controller cache
is a significant contributor to the overall performance of the storage array. The use of the
cache increases controller performance in several ways:
The cache acts as a buffer so that host and drive data transfers do not need to be
synchronized.
If write-back caching is used, the host can send subsequent write commands before the
data from a previous write operation has been written to a drive.
The data for a read or write operation from the host may already be in the cache from a
previous operation, thus eliminating the need to access the drive. This is referred as
"Reach Cache".
If cache pre-fetch is enabled, sequential read access is optimized as cache pre-fetch,
which makes it much more likely that a read operation will find its data in cache rather than
have to read it from a disk drive. This is also referred as "Read Ahead".
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IBM Midrange System Storage Hardware Guide