HP 418371-B21 - Dual Port 72 GB Hard Drive, Technology Brief, Page: 3 / 14

HDD interface technologies
Since the days of Advanced Technology Attachment (ATA, also known as Integrated Drive Electronics
or IDE), the server industry has transitioned through several HDD interface technologies:
•
Small computer system interconnect (SCSI)
•
Serial Attached ATA (SATA)
• Serial Attached SCSI (SAS)
Table 1 lists the key characteristics of these HDD interface technologies.
Table 1. Comparison of HDD interface technologies
SCSI SATA SAS
Transfer/connection type Parallel/shared bus Serial/point-to-point Serial/point-to-point
Current bandwidth [1] 320 MB/s 3.0 Gb/s (300 MB/s) 3 GB/s (300 MB/s)
Future
bandwidth growth? No Yes, to 6 Gb/s Yes, to 6 Gb/s and 12 Gb/s
# of devices supported per
interface per connection
16 15 [2] 16, 256 [3]
HDD type supported SCSI SATA SAS and SATA
Relative reliability Good Adequate Very good
Best suited for Enterprise servers
(replaced by SAS)
Entry level servers Enterprise servers
NOTES:
[1] Actual data rates are slightly lower due to protocol overhead.
[2] Through the use of a SATA port multiplier
[3] Through the use of SAS port expanders
As Table 1 suggests, the SAS interface offers the best solution for the enterprise environment and has
emerged as the preferred choice for high input/out applications.
Key HDD design parameters for enterprise environments
Enterprise-class HDDs must provide maximum performance under a 100 percent duty cycle and
continuous I/O workload in a high-vibration environment. While Mean Time Before Failure (MTBF) is
used to express the length of HDD life in general, a more meaningful benchmark, the Annual Failure
Rate (AFR), better defines the estimated life of an HDD in the enterprise environment.
The AFR is the relation (in percent) between the MTBF and the number of hours that the device is
expected to run per year (100 percent duty cycle = 8760 hours per year). For example, for an
enterprise HDD with an MTBF of 1,200,000 hours, the AFR is calculated as follows:
1,200,000 hours/8760 hours = 136.9863 years, then (1 failure/136.9863 years) x 100% = AFR of 0.73%
An AFR of 0.73 percent means that 0.73 percent of the population of HDDs can be expected to fail
in the average year. In other words, in a system of 100,000 drives, 730 could be expected to fail.
The AFR is less applicable for smaller systems but is meaningful for high-density infrastructures with
thousands or hundreds of thousands of drives. SAS drives, particularly small form factor (SFF) SAS
drives, typically have lowest AFR
3
.
3 The AFR calculations given are for illustration purposes only. The actual failure rate experienced could vary
depending on manufacturing deviations, material quality, and the actual application environment, among other
factors.
3