Getting Technical:  SAS vs. SCSI

September 25 2009

If you’re in the market for a server or a similar device involving highly reliable disk storage, you may be learning more about the relative merits of two types of systems — SATA and SAS (or SCSI) — than ever expected in your wildest dreams.

Servers and other storage devices (such as near-line back-up systems) consist of multiple hard drives linked together. A specialized card known as a RAID controller oversees the flow of information between these drives and with the rest of your server and network in a system known collectively as a “bus,” short for “bidirectional universal switch.” The bus’s job is to keep traffic moving around your system as swiftly and reliably as possible.

If you’re purchasing a system today, you’ll find two major bus technologies vying for your ridership: SAS (Serial Attached SCSI) and SATA.

SAS or Serial Attached SCSI (SCSI, pronounced “scuzzy,” is short for “Small Computer System Interface) has been the gold standard for business-class machines since 1979. Its speed and smarts make it the high-end choice: SAS drives spin faster and hold more cached data, improving performance; SAS devices move data at speeds up to 300MB/sec per port and allow attachment of thousands of devices (important in large server systems). They connect with internal and external devices, have optional redundant cabling (for extra reliability), and allow cable lengths of up to 8 meters.

SATA (Serial Advanced Technology Attachment) has long been known as the cheaper, slower cousin to SAS. While it remains slower overall and less attractive for server systems, SATA’s data transfer rates now rival SAS’s 300 MB/sec, making it appealing for storage applications. SATA drives hold vast amounts of data for less money than SAS; they come in both internal and convenient external (eSATA) configurations. Cable lengths for SATA max out at 2 meters.

Though it sounds ideal, mixing and matching doesn’t really work: if you want to use both SAS and SATA devices, you will need separate controllers for each. Both bus types allow for “hot-plugging” (swapping out devices while the system is online).

What’s the take-home? For inexpensive storage of large volumes of data, SATA systems are hard to beat. Though their attractive prices makes them a tempting rival to SAS for server applications, SAS’s speed, reliability, and extra features still give it the edge in business-class applications.