Dell POWERVAULT MD1000 - Manual (Page 5)

A REFERENCE GUIDE FOR OPTIMIZING DELL™ MD1000 SAS SOLUTIONS VER A00
PAGE 5 5/06/2005
The paper presumes that the reader has a basic understanding of different RAID levels and
interconnect technologies – such as Serial Attached SCSI (SAS) and PCI Express. In this paper,
we will focus on Serial Attached SCSI storage technology, specifically for Dell’s PowerVault
MD1000 storage expansion enclosure and Dell’s PERC 5/E RAID controller. Other storage
technologies such as SCSI or Fibre Channel, as well as topologies such as Network Attached
Storage (NAS) or Storage Area Network (SAN) are outside the scope of this document. All storage
applications will be treated independent of any specific host system or server.
Serial Attached SCSI (SAS)
The SCSI (Small Computing System Interface) protocol originated more than 20 years ago and is
used in the majority of server storage needs (e.g. storage that is internal to a server or physically
connected to a server in an external disk expansion enclosure). The market prevalence of SCSI
has created economies of scale making it one of the most affordable storage interconnect
technologies available today. Many businesses rely on SCSI physical disks to help deliver
extremely cost-effective and reliable storage. In fact, SCSI physical disks have advanced over the
years offering increased performance and more sophisticated features with each iteration.
However, given the inherent limitations of its parallel architecture, the current version of SCSI
physical disks, U320 or Ultra320, will be the last version of the traditional SCSI technology.
Serial Attached SCSI (SAS) is a major iteration of SCSI technology and introduces a host of
benefits when compared to older SCSI technology including,
Dramatic improvement in bandwidth performance
•
The first generation of SAS will support 3.0 Gbps (or 300 MB/sec) per “lane”
• Ultra320 SCSI is the shared-bus architecture. SAS introduces a point to point topology
enabling systems with many physical disks to scale bandwidth far beyond the bandwidth
capacity of Ultra320 SCSI.
Figure 1: Parallel U320 SCSI bus versus point to point Serial Attached SCSI connection
•
SAS introduces the concept of port aggregation to storage interconnects. External storage
devices can connect via a x4 (“by four”)wide-lane. Cabling aggregates four SAS lanes, each
operating at 3.0 Gbps, for a full external connection bandwidth of up to 12.0 Gbps on a single
connector.
Up to 3.0 Gbps per link
Up to320 MBps