Technology considerations in selecting a direct attached storage solution for HP ProLiant Gen8 servers

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SAS/SATA link speed specifications and performance
The true performance of a drive is determined by how quickly the data can be accessed. In the case
of rotational media, that includes time for the requested bits to spin under the head―which is
governed by the latency of track positioning, the areal density of the media, and the RPM of the drive.
However, the speed of the interface electronics is often much greater than the speed of mechanically
locating data on spinning media.
The 6 Gb/s interface has an effective maximum theoretical bandwidth of 600 MB/s. Current SAS
disk drives with high areal density and spinning at 15,000 RPM are capable of a maximum sustained
throughput of about 200 MB/s for sequential data read access. That is less than one-third the
bandwidth of the 6 Gb/s link. Random read and write performance can be significantly lower. When
used in configurations with single disk drives connected directly to the controller, the 6 Gb/s interface
and 6 Gb/s drives provide little or no performance advantage over 3 Gb/s. In these cases, the
performance limiter is the disk drive throughput, not the link speeds.
The performance benefits of 6 Gb/s components become important when constructing SAS fabrics
and larger drive arrays using SAS expanders that support the 6 Gb/s link. These configurations can
combine the I/O load of multiple drives onto a single link and take advantage of the additional
bandwidth as shown in Figure 1. The controller and drives negotiate the link rate based on the
maximum common rate supported by all devices from controller to drive.
Figure 1: The 6 Gb/s interface increases throughput for multiple drives on a port.
The 6 Gb/s SATA and SAS bandwidth is also important when you consider the emerging category of
solid state drives (SSDs). We expect new generations of SSDs to support 500 to 600 MB/s
throughput and be able to consume the entire bandwidth of a single 6 Gb/s link.
Drive controller technology
The drive controller helps determine the capacity, performance, and features available in a server
storage system. Controller choices include embedded chipsets, PCIe host bus adapter (HBA) cards,
and HP Smart Array Controllers. Our drive controllers are available with a variety of internal and
external ports and features such as write cache and RAID arrays.