Performance factors for HP ProLiant Serial Attached Storage (SAS)
3
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 [1] SATA SAS
Transfer/connection type Parallel/shared
bus
Serial/point-to-point Serial/point-to-point
Current bandwidth [2] 320 MB/s 3.0 Gb/s (300
MB/s)
6 Gb/s (600 MB/s)
Future bandwidth growth No Yes, to 6 Gb/s Yes, 12 Gb/s
# of devices supported
per interface per
connection
16 15 [3] 16, 256 [4]
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[1] SCSI data provided for historical reference only.
[2] Actual data rates are slightly lower due to protocol overhead.
[3] Through the use with a SATA port multiplier
[4] Through the use with 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 Input/Output (I/O) workload in a high-vibration environment. While some have used the
term Mean Time Between Failures (MTBF) 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 a
continuous, high workload environment.
The AFR is the relation, expressed as a percentage, 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,
you can calculate the AFR for an enterprise HDD with an MTBF of 1,200,000 hours 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 we can expect 0.73 percent of the population of HDDs to fail in
the average year. In other words, in a system of 100,000 drives, we could expect 730 to fail. (These
AFR calculations are for illustration purposes only. The actual failure rate could vary depending on manufacturing
deviations, material quality, and the actual application environment, among other factors.)