There are many reasons to go straight for a SAS-based RAID controller card. The first and most important is the compatibility of these host adapter cards, as the SAS interface supports the necessary protocols to also handle SATA hard drives, and both use the same basic connectors. SAS stands for Serial Attached SCSI, and represents a serial implementation of the traditional parallel SCSI bus protocol (which is known as the SAS SCSI Protocol, or SSP). However, SAS controllers also implement STP, the SATA Tunneling Protocol, allowing controllers to operate either SAS or SATA drives. In contrast, no SATA controller supports SAS drives.
Why Choice Is So Important
If you are familiar with the storage market, then you already know that there are mainstream drives available in capacities of up to 1. 5 TB, while professional drive models go up to 300 GB (with 600 GB coming soon). The professional drives are always based on 10,000 RPM or 15,000 RPM spindle speeds (or flash memory), use the 2.5” or 3.5” form factor, and the SAS interface, which allows two distinct links to the controller for performance or redundancy reasons. Drives and controllers are many times more expensive than those used for mainstream desktop machines, making them only attractive for business and enterprise applications where high performance may be important enough to justify the cost.
However, large storage capacities for near-line and offline storage can only be realized through high-capacity drives. Examples of near-line storage use are daily backups, while offline storage refers to archiving and similar storage applications where the data doesn’t require fast or direct access. Large capacities require mainstream drives with SATA interfaces, which most of the drive manufacturers modify to fit the durability and reliability requirements of enterprise applications. Hence, Hitachi has its Ultrastar 7K1000, Seagate offers the Barracuda ES, etc.
If you want to combine high performance and high capacity in a single architecture, you have to purchase high capacity storage based on Serial ATA and high performance drives based on SAS. The controllers we reviewed do exactly this.
Where SAS Is Really Impressive
SAS has multiple advantages over SATA. Both serial standards are based on point-to-point interconnects, but SAS supports expanders to allow more drives to be attached. Think of these as switches for SAS. Up to two edge expanders for up to 128 drives each can be connected to a SAS controller, and fan-out expanders are used to connect up to 128 edge expanders. SAS cables are available for individual connects (SFF-8482 cables), but most controllers come with SFF-8470 ports, which route four SAS connectors to backplanes and similar devices. These are also used by InfiniBand.
While SATA is based on half-duplex communications, SAS is a full-duplex link, also supporting port aggregation and dual-link mode to increase performance or to create redundant connectivity.
Now let’s look at Highpoint’s sub-$150 SAS offering.