SAS vs. SATA – Choose Wisely…

SAS and SATA are the two big names in data storage with most new servers and workstations supporting both. Both also come in a 3.5-inch and the ever more popular 2.5-inch form factor—both hot-plug. But what exactly are the differences between the two? For the purposes of this discussion we will only be looking at disk based drives, not the solid state drives—that’s another story.

Choose wisely when deciding on a hard drive for your specific application.

Of the two, SAS or Serial Attached SCSI, provides the most enterprise features, performs better under rigorous conditions, and are more reliable than SATA, which stands for Serial Advanced Technology Attachment. NL-SAS fits somewhere in between the two and is actually a SATA drive with a SAS controller and the NL stands for Near Line, but I digress.

SATA drives are less expensive and offer significantly more storage on a dollar per GB basis ($/GB). SAS wins in the $/IOPS category. So as the old knight in Indiana Jones and the Last Crusade said when Indie is tasked with finding the correct chalice, “Choose wisely…”

There are several different kinds of SATA drives— version 1, 2, 3, 3.1 and 3.2—not to mention a PCI express, and SSD version which we won’t get into right now. Higher revision numbers account for improvements to the performance of the drive and connector cables. SATA 1 can transfer about 1.5 GB/s, while SATA 3 can have a sustained read-write speed of up to 6GB/s which is equivalent to SAS at the lower end of the performance scale.

The SATA drive features individual connectors for data and power, while a SAS drive has a continuous connector for data and power. This means it’s possible to connect a SAS drive to a SATA controller using a continuous (SAS) connector, but a SAS drive cannot be connected to a SATA controller using a SATA connector. SAS cables also have a higher voltage than SATA cables and can measure more than 30-feet in length compared to only about three feet with SATA cables.

The image above shows a 2.5-inch SAS drive stacked on top of a 3.5-inch SATA drive. Notice the break between the brass contacts on the SATA Connector. One for power, one for data.

Later generation drives that support faster speeds can also be limited if connected to an earlier generation port that doesn’t support the faster access speeds. For example if a drive that supports 6GB/s is connected to a 6GB/s port, you get a throughput speed of about 550/520MB/s. Alternatively, if the same drive is connected to a SATA 3GB/s port, throughput is reduced by roughly half so make sure your controller is matched to handle the performance of your drive.

As SATA and SAS drive performance has improved so has throughput.

What, you may be asking yourself, is throughput?

Throughput is proportionally related to bandwidth. Bandwidth is the theoretical maximum amount of data that can travel through a “channel,” and throughput is a measure of how much data actually travels through the “channel” successfully. Think of bandwidth as theoretical value and throughput as a real value. Throughput is also based on drive speed, which is measured in RPMs with platter-based drives. SATA drives run in the neighborhood of 5400 to 7200 RPM with newer SATA drives capable of 10K RPMs. SAS drives operate in the range of 7200 to 15000 RPMs. Rotational speeds can also be looked at as mechanical latency. The faster the drive rotates, the faster the read-write speeds, and the greater the throughput. The slower the drive rotates, the slower the read-write speed and reduced throughput—simple right?

“Think of bandwidth as a theoretical value
and throughput as a real value.”

Of course throughput performance is also dependent on the type of connector you’re using which can be different. With SATA connectors you can expect a throughput rate of about 150 MB/s with SATA 1 and up to 600MB/s with SATA III. SAS delivers a maximum throughput supported by the interface of 1,200 to 1,500 MB/s. Of course if you purchase a drive that supports the maximum transfer rate and connect it to an older generation port it won’t be able to perform at its rated speeds without the next generation controller, remember that example above?

On the other hand, NL-SAS, or Near Line SAS, is an enterprise-class SATA drive that supports most of the enterprise features found on SAS drives including duplexing and hot-plug access. However, this is still a SATA drive that uses a SAS interface, but still doesn’t have the same performance as a SAS drive. In other words, still at the 5400 to 7200 RPMs and still the same rate of failure, but you will benefit from the SAS interface increasing IOPS.

In general, SAS drives are set up for full duplexing, which means each port can read and write data at the same time. In general, SATA drives, are only capable of reading or writing, but not both at the same time—the exception being NL-SAS.

“…that extra 10 multiplier makes
a big difference.”

SATA typically offers more storage than SAS, but is significantly slower and has a higher rate of failure. Aside from NL-SAS, which in many ways is a SATA drive on steroids, SATA doesn’t offer the same error correcting features found on the SAS drives. At lower storage capacities SAS is just slightly more expensive than SATA, but as storage space increases, SAS becomes significantly more expensive.

Bit Error Rate (BER) is a measure of how often bit errors may occur on the media. On a SAS drive the error rate is about 10^16 or a 10 with 16 zeros behind it. SATA drives have a BER of 10^15, which seems pretty close on paper, but that extra 10 multiplier makes a big difference.

While hard drive capacities have increased over the years, data failure rates have remained fairly consistent and consumer SATA drives have a much higher failure rate (four orders of magnitude higher) than enterprise-class SAS drives. As a result, if SATA drives are used for storage in large RAID groups and one drive fails, just rebuilding the redundant drive from a mirrored drive can use a significant amount of network resources. Combined with the speed issues and a higher rate of failure of the SATA drives, an increased incidence of silent data corruption and additional backup drive failures is a distinct possibility. As a result, your server can be constantly rebuilding the drives, reducing available resources to your network.

So what does it all mean?

In a nut shell, both serve a purpose on the storage issue. SATA offers affordable high capacity storage for non-critical needs that don’t require fast access and supports a limited number of users. SAS is for performance applications and highly virtualized environments that address a high volume of users and mission critical applications supporting faster IOPS than SATA. NL-SAS falls somewhere in between the two offering an enterprise-class medium with high storage at an affordable price, but not at the same level of performance as SAS. In conclusion, weigh your options carefully to choose the best option for your server environment that will minimize cost, but also provide the best performance for your storage needs.