HP 504062-B21 Performance factors for HP ProLiant Serial Attached Storage (SAS - Page 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 SATA Transfer/connection type Parallel/shared bus Serial/point-to-point Current bandwidth [1] 320 MB/s 3.0 Gb/s (300 MB/s) Future bandwidth growth? No Yes, to 6 Gb/s # of devices supported per 16 interface per connection 15 [2] HDD type supported SCSI SATA Relative reliability Good Adequate Best suited for Enterprise servers (replaced by SAS) Entry level 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 SAS Serial/point-to-point 3 GB/s (300 MB/s) Yes, to 6 Gb/s and 12 Gb/s 16, 256 [3] SAS and SATA Very good Enterprise servers 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 AFR3. 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