Serial attached SCSI: Who dares wins
research Serial Attached SCSI or SAS is a communication protocol for direct attached storage designed to allow much higher speed data transfers than traditional SCSI (small computer system interface).
SAS uses a serial communication method (i.e., like USB and Firewire) instead of a parallel method (i.e. like SCSI). As the successor to SCSI, SAS retains the SCSI commands for interacting with devices.
SAS drives are built for high-performance/high-availability use and are comparable (in price and performance) to FC drives used in enterprise class storage. First-generation SAS interfaces have a throughput of 3Gb/s, with 12Gb/s projected by 2010. SAS uses multiple point-to-point connections, enabling highly scalable fault-tolerant designs. Like FC disks, SAS disks are designed for environments where data transactions are high and data availability is essential.
SATA, on the other hand, is primarily designed for low cost-per-gigabyte bulk storage where transaction rates are low and data availability is not mission-critical. As a result, SATA drives feature lower spindle speeds and lower mean-time-between-failure rates than SAS or FC drives but at a much lower cost per gigabyte.
SAS makes larger storage topologies more practical through larger addressing, greater throughput and simpler cabling. It replaces the wide 68-pin ribbon SCSI cables with a single thin 7-wire SAS cable of up to 8 metres. This enables compact design, improves airflow inside enclosures and simplifies hot-plug connections. These features enable the development of large scale storage enclosures at a lower cost to FC alternatives.
SAS has been designed for compatibility with SATA, enabling SAS and SATA drives to be mixed on the same SAS controller. SATA drives may be plugged into SAS controllers and communicate on the same physical cable as SAS disks. However, SAS disks may not be plugged into a SATA controller. By allowing high performance SAS drives to be mixed and matched with low cost SATA drives, SAS provides an enabling technology for storage vendors to create highly flexible storage arrays that can optimise data layout to provide the right mix of performance, availability and cost in a single enclosure.
SAS market adoption
Serial ATA will continue as the prevalent disk interface technology in desktop PCs, sub-entry level servers and subentry level networked storage systems (NAS and SAN) that store reference
type data where cost is a primary concern.
Through 2006/7, SAS will progressively replace SCSI as the preferred internal storage for entry level and midrange servers. When used in conjunction with 2.5 inch drives, SAS will become a viable option for internal storage in blade servers. In these segments SAS is a sustaining technology and will not precipitate a shift in storage vendor's market shares.
The primary competitor to SAS in this market will be from SATA as it continues to improve in performance; however lack of redundancy will limit its adoption in mission critical servers.
SAS has the potential to be highly disruptive in the midrange networked storage market (i.e. midrange SAN and NAS). With simpler interconnection and highly scalable topologies (both in capacity and performance), SAS enables vendors to build networked storage systems at a significantly lower cost to equivalent FC based systems. With a lower cost enclosure/backplane and the capability to blend high performance SAS with low cost SATA, storage vendors will be able to create low cost systems with dynamic price/performance characteristics that will challenge the current FC disk based devices (eg EMC, Hewlett-Packard, Network Appliance).
Unless current market leaders create competitive products by 2007, I expect low cost new entrants to successfully challenge the low end of the midrange network storage market. Either way, this market will further commoditise though 2007/8.
Before considering these devices, clients must first understand the storage requirements of their applications/datasets and identify those instances were a hybrid SAS/SATA device can satisfy requirements at a lower cost to FC alternatives. Failure to do so will only lead to the introduction of another storage vendor or architecture, which increases complexity, without measurable benefits.
It is important to realise the lower cost of SAS arrays not because SAS disk is cheaper than FC disk. The lower costs are realised though the simpler interconnect, which makes the enclosure cheaper, and blending of SAS and SATA in the same enclosure, which yields a lower cost per Gigabyte. In these networked storage devices SAS will only be used as a device interface and will not replace FC or iSCSI as the dominant SAN interconnect.
High-end, enterprise class SANs will continue to be built around FC drives. Without the need for mix and match with the low cost, lower performance SATA drives, there is insufficient benefit to force a migration away from FC drives given established vendors considerable existing investment in the R&D and manufacturing of FC based devices.Conclusion
While the introduction of Serial Attached SCSI (SAS) will have a significant impact on the storage environment though 2006/7, over the next 12 months clients should be wary of the hype
vendors will use to promote it. By year end 2005, technical staff should gain a basic understanding of the key features/benefits of SAS. Though 2006/7, IT organisations should begin using
SAS, in conjunction with SATA, in DAS, SAN and NAS configurations when it provides a lower cost storage alternative [i.e. than Fibre Channel (FC)] while still meeting application and data
service level requirements.
biography
Dr Kevin McIsaac has over 20 years of IT experience and is a recognised expert in infrastructure, operations, vendor management and the art of running IT as a business. For the last five years, Dr McIsaac worked for META Group, most recently as the Research Director Asia-Pacific, researching, distilling and disseminating best practices in IT. In that role he advised the CIOs and
the IT management teams of leading Asia-Pacific organisations. Prior to joining META Group, Dr McIsaac held leadership positions at Computer Associates and Functional Software.
© 2005 IBRS Pty Ltd. Written permission is required from IBRS Pty Ltd before this publication may be reproduced in any form.