SAS: When SATA Is Not Enough
Have a look at today's motherboards (or even some of the older platforms out there). Is it really still necessary to buy a dedicated RAID controller? Three-gigabit SATA ports are found on pretty much every board, just like audio and networking connectivity. The most modern chipsets, like AMD's A75 and Intel's Z68 even incorporate SATA 6Gb/s support. Backed by reliable power circuitry, a powerful processor, and plenty of I/O, aren't you already getting the hallmarks of a solid add-in storage card? Where does it make sense to make that investment in a discrete controller?
In most cases, mainstream users are able to configure RAID 0, 1, 5, and 10 arrays using their motherboard's built-in SATA ports and a little bit of software, yielding reasonable performance. But in environments where more advanced RAID levels like 6, 50, or 60 are necessary, beefier disk management is desired, or scalability is needed, those chipset-based controllers come up inadequate. That's when it's time for a professional-class solution.
And at that point, you're no longer limited to SATA storage. A great number of add-in cards facilitate support for Serial-Attached SCSI (SAS) or Fibre Channel (FC) disks, each interface offering unique advantages.
SAS and FC for Professional RAID
Each of the three available interfaces (SATA, SAS, and FC) brings different strengths and weaknesses to the table; none of them can be definitively labeled the best. The strengths of SATA-based drives include some of the highest capacities and low cost, while still managing great data rates. SAS disks generally emphasize reliability, scalability, and high I/O rates. FC storage focuses on continuous, fast data rates. As a legacy solution, some enterprises still use Ultra SCSI, although it's hampered by a maximum device count of 16 (which includes one controller and up to 15 disks). Moreover, its maximum aggregate bandwidth of 320 MB/s (in the case of Ultra-320 SCSI) is quite paltry compared to its successors.
Ultra SCSI used to be the standard for professional, enterprise storage solutions. SAS has, however, largely taken over now, offering not only significantly higher bandwidth, but also the flexibility to accommodate mixed SAS/SATA environments to really optimize cost, performance, dependability, and capacity even within a single JBOD. Additionally, many SAS disks have two ports for the purpose of redundancy. If one controller card goes out, connecting the drive to a second controller enables fail-over. Thus, SAS can support high-availability setups.
Furthermore, SAS is not merely a point-to-point protocol between a controller and a storage device. It supports up to 255 storage devices per SAS cable via expanders. By using a two-tiered SAS expander structure, theoretically 255 x 255 (or slightly more than 65 000) storage devices could be connected to a single SAS channel, assuming the controller chip supports such a large quantity internally.
Adaptec, Areca, HighPoint, and LSI: Four SAS RAID Controllers Tested
In this comparison test, we're scrutinizing the performance of current SAS RAID controllers, represented by four products: Adaptec's RAID 6805, Areca's ARC-1880i, HighPoint's RocketRAID 2720SGL, and LSI's MegaRAID 9265-8i.
Why SAS and not FC? On one hand, SAS is the more interesting and relevant architecture. It offers features like zoning that can be highly attractive for professional use. On the other hand, market data shows that FC’s role in the professional storage market is declining, and some analysts even predict its demise based on the number of shipped hard disks. While the future of FC seems bleak, IDC predicts that SAS hard disks will claim 72 percent share of the enterprise hard disk market in 2014.