Maxtor D740X-6L 20.0 AT, Руководство по эксплуатации продукта

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Basic Principles of Operation
5-11 Maxtor D740X-6L 20.0/40.0/60.0/80.0GB AT
transferring data requested by the host. Unlike a noncaching controller, Maxtor’s disk
controller continues a read operation after the requested data has been transferred to
the host system. This read operation terminates after a programmed amount of
subsequent data has been read into the cache segment.
The cache memory consists of a 1.9MB DRAM buffer allocated to hold the data,
which can be directly accessed by the host by means of the READ and WRITE
commands. The memory functions as a group of segments with rollover points at the
end of cache memory. The unit of data stored is the logical block (that is, a multiple
of the 512 byte sector). Therefore, all accesses to the cache memory must be in
multiples of the sector size. Almost all non-read/write commands force emptying of
the cache:
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When a write command is executed with write caching enabled, the drive stores the
data to be written in a DRAM cache buffer, and immediately sends a GOOD
STATUS message to the host before the data is actually written to the disk. The host
is then free to move on to other tasks, such as preparing data for the next data transfer,
without having to wait for the drive to seek to the appropriate track, or rotate to the
specified sector.
While the host is preparing data for the next transfer, the drive immediately writes the
cached data to the disk.
WriteCache allows data to be transferred in a continuous flow to the drive, rather than
as individual blocks of data separated by disk access delays. This is achieved by taking
advantage of the ability to write blocks of data sequentially on a disk that is formatted
with a 1:1 interleave. This means that as the last byte of data is transferred out of the
write cache and the head passes over the next sector of the disk, the first byte of the
of the next block of data is ready to be transferred, thus there is no interruption or
delay in the data transfer process.
The WriteCache algorithm fills the cache buffer with new data from the host while
simultaneously transferring data to the disk that the host previously stored in the cache.
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In a drive without DisCache, there is a delay during sequential reads because of the
rotational latency, even if the disk actuator already is positioned at the desired cylinder.
DisCache eliminates this rotational latency time (4.17 ms on average) when requested
data resides in the cache.
Moreover, the disk must often service requests from multiple processes in a
multitasking or multiuser environment. In these instances, while each process might
request data sequentially, the disk drive must share time among all these processes. In
most disk drives, the heads must move from one location to another. With DisCache,
even if another process interrupts, the drive continues to access the data sequentially
from its high-speed memory. In handling multiple processes, DisCache achieves its
most impressive performance gains, saving both seek and latency time when desired
data resides in the cache.
The cache can be flexibly divided into several segments under program control. Each
segment contains one cache entry. A cache entry consists of the requested read data
plus its corresponding prefetch data.