Average Access Time

Average Access Time
Access time is the metric that represents the composite of all the other specifications reflecting
random performance positioning in the hard disk. As such, it is the best figure for assessing overall
positioning performance, and you'd expect it to be the specification most used by hard disk
manufacturers and enthusiasts alike. Depending on your level of cynicism then, you will either be
very surprised or not surprised much at all, to learn that it is rarely even discussed. Ironically, in
the world of CD-ROMs and other optical storage it is the figure that is universally used for
comparing positioning speed. I am really not sure why this discrepancy exists.
Perhaps the problem is that access time is really a derived figure, comprised of the other
positioning performance specifications. The most common definition is:
Access Time = Command Overhead Time + Seek Time + Settle Time + Latency
The speed with which data can be transmitted from one device to another. Data rates are often
measured in megabits (million bits) or megabytes (million bytes) per second. These are usually
abbreviated as Mbps and MBps, respectively.
Buffer Size（Cache）
A small fast memory holding recently accessed data, designed to speed up subsequent access to
the same data. Most often applied to processor-memory access but also used for a local copy of
data accessible over a network etc.
When data is read from, or written to, main memory a copy is also saved in the cache, along with
the associated main memory address. The cache monitors addresses of subsequent reads to see if
the required data is already in the cache. If it is (a cache hit) then it is returned immediately and
the main memory read is aborted (or not started). If the data is not cached (a cache miss) then it is
fetched from main memory and also saved in the cache.
The cache is built from faster memory chips than main memory so a cache hit takes much less
time to complete than a normal memory access. The cache may be located on the same integrated
circuit as the CPU, in order to further reduce the access time. In this case it is often known as
primary cache since there may be a larger, slower secondary cache outside the CPU chip.
The most important characteristic of a cache is its hit rate - the fraction of all memory accesses
which are satisfied from the cache. This in turn depends on the cache design but mostly on its size
relative to the main memory. The size is limited by the cost of fast memory chips.
The hit rate also depends on the access pattern of the particular program being run (the sequence
of addresses being read and written). Caches rely on two properties of the access patterns of most
programs: temporal locality - if something is accessed once, it is likely to be accessed again soon,
and spatial locality - if one memory location is accessed then nearby memory locations are also
likely to be accessed. In order to exploit spatial locality, caches often operate on several words at a
time, a "cache line" or "cache block". Main memory reads and writes are whole cache lines.
When the processor wants to write to main memory, the data is first written to the cache on the
assumption that the processor will probably read it again soon. Various different policies are used.
In a write-through cache, data is written to main memory at the same time as it is cached. In a
write-back cache it is only written to main memory when it is forced out of the cache.
If all accesses were writes then, with a write-through policy, every write to the cache would
necessitate a main memory write, thus slowing the system down to main memory speed. However,
statistically, most accesses are reads and most of these will be satisfied from the cache.
Write-through is simpler than write-back because an entry that is to be replaced can just be
overwritten in the cache as it will already have been copied to main memory whereas write-back
requires the cache to initiate a main memory write of the flushed entry followed (for a processor
read) by a main memory read. However, write-back is more efficient because an entry may be
written many times in the cache without a main memory access.
When the cache is full and it is desired to cache another line of data then a cache entry is selected
to be written back to main memory or "flushed". The new line is then put in its place. Which entry
is chosen to be flushed is determined by a "replacement algorithm".
Some processors have separate instruction and data caches. Both can be active at the same time,
allowing an instruction fetch to overlap with a data read or write. This separation also avoids the
possibility of bad cache conflict between say the instructions in a loop and some data in an array
which is accessed by that loop.
Noise & Temperature
It comes from motor. So motor is the key to reduce the noise and temperature. If you can keep the
temperature of hard disk down, then you can keep your hard disk effective.
3.Physical structure of hard disk
HD consists of platter, control circuit board and interface parts.
A hard disk is a sealed unit containing a number of platters in a stack. Hard disks may be mounted
in a horizontal or a vertical position. In this description, the hard drive is mounted horizontally.
Electromagnetic read/write heads are positioned above and below each platter. As the platters spin,
the drive heads move in toward the center surface and out toward the edge. In this way, the drive
heads can reach the entire surface of each platter.