Exploiting Disk Layout and Block Access History for I/O Prefetch - Advanced Operating Systems and Kernel Applications - page 202

Figure 1. Performance gap between processors and disks.

introduction

of rotating disk platters. These data can be read
or written through the disk head attached on the
moving disk arms. In general, accessing one
data block involves three major operations, each
of which causes a delay accordingly. When the
hard disk receives a request (read/write) to data
on a certain location, the disk arm must first po-
sition to the correct disk track where the data is
located. This operation results in a seek latency .
Then the disk head has to wait until the disk
platters rotate to the correct position where the
target data block is right beneath the disk head,
which causes a rotational latency . Finally, data
transfer can be started from or to the disk platter
surface, depending on the operation type (read/
write), which leads to a transfer latency . These
three types of latency together form the aggregate
latency of servicing a disk request. Since seek and
rotational latencies are essentially determined
by the speed of mechanic parts, the first two
operations usually account for a large portion of
the aggregate service latency to complete a disk
access and should be minimized.
The performance of the hard disk is highly
dependent on the workload access pattern, i.e.

As the Moore's law states, over the last three
decades the processor speed doubles every 18
months, which brings a steady performance im-
provement at an exponential rate. In contrast, the
access time of the hard disk, an electro-mechanical
device, has been improved at a much slower
pace, only around 8% per year (Gray & Shenoy,
2000). As a result, the performance gap between
processors and hard disks is increasingly widen-
ing and this trend will continue in the future. As
shown in Figure 1, in 1980 each disk access costs
around 87,000 CPU cycles only, while this number
grows to 5,000,000 cycles in 2000 (Bryant and
O'Hallaron 2003). In other words, relative to the
processor speed, the hard disk is becoming 57
times slower during the twenty years. Such an ever-
growing performance gap between the processor
and the hard disk strongly indicates that, the disk
performance is becoming the key bottleneck of
overall system performance.
The excessively high access latency of the
hard disk essentially stems from its mechanic
nature. Hard disk drives store data on the surface