HP 418371-B21 - Dual Port 72 GB Hard Drive User manual

Performance factors for HP ProLiant
Serial Attached Storage (SAS)
technology brief
Abstract.............................................................................................................................................. 2
Introduction......................................................................................................................................... 2
HDD controller technologies .............................................................................................................. 2
HDD interface technologies ............................................................................................................... 3
Key HDD design parameters for enterprise environments ...................................................................... 3
SAS use for enterprise systems............................................................................................................... 4
HDD performance comparisons............................................................................................................. 5
Performance comparison between SATA and SAS drives...................................................................... 5
Performance comparisons between SAS drives .................................................................................... 8
Performance comparisons based on form factor............................................................................... 8
Performance comparisons based on platter speed .......................................................................... 11
SAS drive specifications ..................................................................................................................... 13
Conclusion........................................................................................................................................ 13
For more information.......................................................................................................................... 14
Call to action .................................................................................................................................... 14

2
Abstract
Enterprise-class hard disk drives (HDD) must meet the maximum reliability and scalable performance
goals of the 24/7 enterprise environment. Serial Attached SCSI (SAS) has become the defacto 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 comparison information of
the latest small form factor SAS drives available from HP at time of publication.
Introduction
The mission-critical, 24/7 enterprise environment places stringent demands on storage technology.
Reliability and performance are of paramount importance. While server downtime can be costly, even
slow server performance can increase operating costs. Hard disk controllers, interfaces, and drives
have evolved to address the reliability and performance issues of the enterprise storage system.
HDD controller technologies
To enhance the operating efficiency of multiple drives, HDD controllers use logic-based solutions
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 avoid the use of slower
system memory. 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 strategies1are 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 – Mirroring and striping for redundancy and performance improvement
•RAID 1+0 (10) – Mirroring and 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 6 – Striping with dual parity for improved performance and fault tolerance2
The choice of RAID strategy depends on the desired balance of protection and performance, along
with the number of hard drives available.
1For more information about disk drive technology and RAID refer to the HP technology brief titled “Disk Drive
Technology Overview” available at
http://h20000.www2.hp.com/bc/docs/support/SupportManual/c01071496/c01071496.pdf .
2For more information about RAID 6 technology refer to the HP technology brief titled “RAID 6 with HP ADG
Technology” available at
http://h20000.www2.hp.com/bc/docs/support/SupportManual/c00386950/c00386950.pdf

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 AFR3.
3The 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

Another important factor to HDD performance is seek time, described as the time from when a read or
write action is initiated until the data transfer from or to the disk actually begins. The smaller platters
of SFF SAS drives inherently yield lower seek times, an advantage in file servers with frequent random
accesses.
SAS use for enterprise systems
This section describes SAS design features that affect the performance and operating efficiency of
SAS HDDs for enterprise-class systems. For a general discussion of SAS technology, refer to the HP
white paper “Serial Attached SCSI Technology” available at the following HP web page:
http://h20000.www2.hp.com/bc/docs/support/SupportManual/c00302340/c00302340.pdf
Since RAID strategies and AFR numbers favor high-density systems, SFF SAS drives are an attractive
solution for large enterprise systems. Compared to 3.5-inch large form factor (LFF) SAS drives,
2.5-inch SFF SAS drives have these advantages:
•Lower AFR
•15 to 20 percent higher Input/output Operations Per Second (IOPS)
•30 percent smaller physical size
•Up to half the power consumption
Increasing HDD density in a RAID-based system invariably enhances the overall performance of the
storage system. Increasing the drive density also expands the choice of RAID strategies as indicated in
Table 2.
Table 2. Comparison of drive support in two 1U HP ProLiant servers
Server DL320 G5 DL360 G5
Drive type LFF SAS [1] or SATA SFF SAS or SATA
# of drives 4 6
RAID support RAID 0, 1 [1] RAID 0, 1, 10, 5, 6 [2]
NOTES:
[1] Requires optional SAS controller
[2] RAID 6 requires P400i with 512MB Battery Backed Write Cache option
At the drive level, smaller/faster is usually better. SFF drives offer a spindle-per-U advantage that
yields better performance in most systems.
Traditionally, LFF drives have a cost-per-GB advantage over SFF types. For workstation and small-to-
medium business customers, getting the most storage capacity for the money is a key factor in setting
up a system. For the enterprise IT architect, however, the goal should be to get the most work (that is,
more IOPS) out of the system, and this is best achieved with high density storage systems.
4

HDD performance comparisons
This section provides performance results of comparisons between HDD interface types, form factors,
and platter speeds. Other factors such as operating system, application environment, controller type,
RAID strategy, and general workload also play a part in system performance and should be
considered when evaluating systems.
Performance comparison between SATA and SAS drives
Systems that support SAS can be configured with a mix of SAS and SATA drives4, although
performance goals are more easily achieved using SAS drives. Figures 1 through 3 are plots of
performance data from tests of SATA drives (non-NCQ and NCQ) and SAS drives using the following
configuration:
•System: HP StorageWorks 60 Modular Smart Array
•SAS Controller: HP Smart Array P400, firmware version 4.12, 512 MB cache, RAID 1
Figure 1. Relative performance: sequential operations of non-NCQ SATA, NCQ SATA, and SAS
2-drive array with HP Smart Array P400 Controller, RAID 1
Sequential Operations, RAID 1 (128 KB)
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RELATIVE PERFORMANCE (%)
IOMETER 07.30.2004
SATA, Non-NCQ 60-GB, 5.4K, 25/75 R/W cache (baseline performance level)
SATA, NCQ 120-GB, 5.4K, 25/75 R/W cache
SAS, 72-GB, 15K, 25/75 R/W cache
4Mixing SATA and SAS drives within a logical drive is NOT allowed.
5

Figure 2. Relative performance: random operations of non-NCQ SATA, NCQ SATA, and SAS 2-drive
array with HP Smart Array Controller P400 Controller, RAID 1
Random Operations, RAID 1 (128 KB)
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RELATIVE PERFORMANCE (%)
IOMETER 07.30.2004
SATA, Non-NCQ 60-GB, 5.4K, 25/75 R/W cache (baseline performance level)
SATA, NCQ 120-GB, 5.4K, 25/75 R/W cache
SAS, 72-GB, 15K, 25/75 R/W cache
The HP P400 controller supports NCQ, so the NCQ SATA drives provide an increase in performance
over non-NCQ SATA drives. The SAS arrays, however, are much faster than either of the SATA
arrays and exhibit a substantial performance increase for most operations.
Figure 3 compares the throughput performance of the SATA and SAS drives in the same
configuration, and illustrates the substantial improvement of SAS drives.
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Figure 3. Throughput performance: random operations of non-NCQ SATA, NCQ SATA, and SAS
2-drive array with HP Smart Array P400 Controller, RAID 1
Random Opeations, RAID 1 (128 KB)
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THROUGHPUT (IOPS)
IOMETER 07.30.2004
SATA, Non-NCQ 60-GB, 5.4K, 25/75 R/W cache
SATA, NCQ 120-GB, 5.4K, 25/75 R/W cache
SAS, 72-GB, 15K, 25/75 R/W cache
7

Performance comparisons between SAS drives
This section includes comparisons of SAS drives based on form factor and platter speed, along with
differences in RAID strategies.
Performance comparisons based on form factor
Figures 4 through 8 compare the performance of SFF SAS drives and LFF SAS drives. The following
configuration was used for testing the SAS drives:
•System: HP StorageWorks 70 Modular Smart Array
•SAS Controller: HP Smart Array P800, firmware version 4.06, 512 MB cache
Figure 4. Performance comparison: SFF to LFF 15K 72-GB SAS drives with HP Smart Array P800
Controller, RAID 0 (128K)
RAID-0 (128KB)
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% Diff - SFF 15K Performance compared to LFF 15K
IOMETER 07.30.2004
4 SFF SAS drives
6 SFF SAS drives
8 SFF SAS drives
12 SFF SAS drives
Level of performance for LFF SAS drives
As indicated in Figure 4, SFF SAS drive arrays yield better performance in a RAID 0 solution than LFF
SAS drive arrays. While providing maximum storage capacity and performance, RAID 0 does not
provide any disk redundancy.
8

Figure 5. Performance comparison: SFF to LFF 15K 72-GB SAS drives with HP Smart Array P800
Controller, RAID 1 (128K)
RAID-1 (128KB)
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% Diff - SFF 15K Performance compared to LFF 15K
IOMETER 07.30.2004
Level of
p
erformance for LFF SAS drives
2 SFF SAS drives
4 SFF SAS rives
6 SFF SAS drives
8 SFF SAS drives
12 SFF SAS drives
RAID 1 is a strategy for achieving redundancy rather than enhanced performance. Nevertheless, as
indicated in Figure 5, SFF SAS drives provide better performance than LFF SAS drives for most
RAID 1 configurations.
9

Figure 6 illustrates the IOPS performance of various array sizes of SFF SAS drives with the HP Smart
Array P800 Controller using RAID 5 (64K).
Figure 6. RAID5 performance of SFF 15K 72-GB SAS drive arrays
SA-P800, RAID-5 (64KB) - SFF 15K
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THROUGHPUT (IO Per Second)
IOMETER 07.30.2004
4 SFF SAS drives
6 SFF SAS drives
8 SFF SAS drives
12 SFF SAS drives
10