Performance factors for HP ProLiant Serial Attached Storage (SAS)

Introduction

Enterprise-class hard disk drives (HDD) must meet the maximum reliability and scalable performance

goals of the 24/7 enterprise environment. While every minute of server downtime can prove costly,

even a server’s slow performance can increase your operating costs. Hard disk controllers, interfaces,

and drives have evolved to address the reliability and performance issues of the enterprise storage

system.

Serial Attached SCSI (SAS) has become the de facto HDD standard for mission-critical applications.

This paper explains the emergence of SAS and the key parameters of SAS drive technology. It also

includes technical data and comparisons of our latest small form factor SAS drives available at the

time of publication.

HDD controller technologies

HDD controllers use logic-based solutions to enhance the operating efficiency of multiple drives,

including:

 Data buffering and read/write caching

 Queuing control and read/write reordering

 Error management and pre-failure warning

 Redundant Array of Independent Drives (RAID)

HDD controllers employ on-board data buffering and caching techniques to optimize the use of slow

drives. Queuing techniques such as Tagged Command Queuing (TCQ) and Native Command

Queuing (NCQ) allow controllers and compatible HDDs to take advantage of the read/write head

position for more efficient drive operations. RAID is another logic-based solution that places data in

stripes across multiple drives to enhance reliability, performance, and data integrity. A variety of RAID

strategies are in use:

 RAID 0–Striping to two or more disks (no parity) for performance improvement (no redundancy)

 RAID 1–Mirroring data on two disks (no parity) for redundancy, slight performance improvement

 RAID 0+1–Striping and then mirroring for redundancy and performance improvement

 RAID 1+0 (10)–Mirroring and then striping for redundancy and performance improvement

 RAID 3–Striping (byte level) with parity for improved performance and fault tolerance

 RAID 4–Striping (block level) with parity for improved performance and fault tolerance

 RAID 5–Striping with distributed parity for improved performance and fault tolerance

 RAID 50–Striping of RAID 0 with the distributed parity of RAID 5 (RAID 0 array striped across RAID

5 elements requires at least six drives)

 RAID 6–Striping with dual distributed parities for improved performance and fault tolerance

 RAID 60–Striping of RAID 0 with the distributed double parity of RAID 6 (RAID 0 array striped

across RAID 6 elements requires at least eight disks)

The choice of RAID strategy depends on the desired balance of protection and performance, along

with the number of hard drives available.

For more information about disk drive technology and RAID refer to the HP technology brief titled “Server Drive

Technology” available at

http://h20000.www2.hp.com/bc/docs/support/SupportManual/c01071496/c01071496.pdf