While Fibre Channel (FC) as a drive connection type is still available, the trend towards SAS and SATA is so strong that FC won’t be included in this discussion. In this comparison, the differences essentially come down to the capabilities of the interfaces because the same flash substrate can be used with all three connectivity types.


Both SATA (Serial ATA) and SAS (Serial Attached SCSI) are protocols that have evolved from the parallel bus architectures of previous generations. SATA uses an updated version of the ATA command set and is more at home in a general-purpose environment where ease of deployment, performance, and simplicity are the key driving factors.  Although SATA does support external connections, it’s typically found “inside the box” (that is, the target and controller are in the same chassis).


SAS, using an updated version of the SCSI command set, is more often seen in large scale, dual controller storage arrays. It’s also used in external “box-to-box” connections, or in systems requiring a very high degree of dynamic, low-level configuration changes and/or a very detailed status reporting schema.


Last, PCIe devices rely on PCIe for the physical transport (packet) mechanism, but still rely on other protocols for data communication. These devices may also incorporate additional functional logic to merge function blocks more typically found in separate devices, such as HBAs integrated into the controller in the PCIe device.



SATA


Compared with SAS SSDs, SATA offers a number of advantages. SATA has an impressive development track record, from 1.5 gigabit (Gb) through the current 6 Gb bandwidth (doubling with each generation), with 12 Gb on the horizon. It also has a significant adoption advantage over SAS, with roughly twice the penetration in new server ports sold.


Overall, SATA is more economical, based on cost per GB and is more cost-effective, including all the associated components (controllers, etc). It also provides a much smaller implementation overhead, meaning that SATA devices work with both SAS and SATA host connections right out of the box.


However, SATA doesn’t support dual ported host connection (unless an adapter is used), meaning SATA SSDs can’t be used in a failover configuration, via redundant drive connections.



SAS


Serial Attached SCSI is effectively the market successor to fibre channel (FC) in the high performance drive connectivity space. Note that FC drive connectivity is a different discussion than FC implemented as a storage area network (SAN) interface. For both SSDs and rotating drives, SAS does offer two key benefits over SATA, including a much deeper device queue and native dual path support.


However, compared to SATA, SAS is more expensive per GB and is less flexible, as it requires SAS RAID controllers and SAS host interfaces. Additionally, it simply costs more, both for the drive and the host controller.



PCIe


PCIe SSDs are designed along two very different paths. Some designs simply mount a group of SATA SSDs to a card with an integrated RAID controller, whereas others have an ASIC on the card that handles both the host interface and NAND management functions.


Both designs offer a different internal, direct attached storage option when compared with the drive form-factor. While there are external arrays that connect via a cable and PCIe bus card, this discussion will focus on PCIe cards that mount inside the host.


Compared with both SATA and SAS drive form-factor SSDs, PCIe cards offer the lowest potential latency, as they move the flash storage closest to the server CPU-memory complex. Implementation is as simple as installing a server card, but does involve a reboot, which can make upgrades and maintenance more complicated. Additionally, a PCIe SSD card does consume a bus slot and some of a server’s finite PCIe lanes.



Use cases


Although SAS has been the clear choice for external, primary array systems and high performance, tier-one, online storage, this has been traditionally driven by:


  1. BulletSAS rotating drives show higher performance compared to their SATA counterparts

  2. BulletSAS drives support dual data paths to the media, enabling redundant/failover connections


However, modern SATA SSDs dwarf the performance of modern rotating SAS drives, often by two orders of magnitude. The performance advantage SAS has over SATA still remains, but only with rotating drives – not SSDs.


Likewise, there are several adapter devices that can take the single port of a SATA drive and make it into a dual port SAS drive. The adapter boards are commonly included with dual controller storage arrays (when SATA drives are offered).


SATA SSDs are popular as server boot drives, high performance single port ‘in chassis’ drives (logging) and caches in front of rotating drives (SAS and/or SATA). They are also an easy, high-speed upgrade for slower rotating disk storage. In addition, SATA is the protocol choice for ‘client’ implementations, like notebooks and desktops that need better performance than can be provided by spinning disk drives, but for which architectural changes (as SAS would require) are prohibitively expensive.


From an implementation perspective, SATA 6 Gb/s SSDs, like Micron’s P300 may make most sense for the broadest range of applications. This drive can be used with either SAS or SATA hosts and provides potentially the ‘best of both worlds’. Its 6 Gb/s interface provides the highest performance for a SATA device and offers better dollar/capacity economics than SAS. And, although SATA 3 Gb/s is the most common connectivity option in servers, an SSD using the 6 Gb/s interface won’t become the bottleneck when the system is upgraded.


PCIe is the technical choice where highest possible performance and lowest possible latency dominate cost and ease of deployment concerns. It’s well suited as a local performance upgrade for a single server or as the fastest direct attached storage for distributed or isolated high IOPS applications. Logical implementations include Web 2.0, highly interactive databases/database acceleration and applications in video, graphics, simulation, etc. Generally, PCIe SSDs offer high performance and higher capacity per internal footprint.



Bottlenecks


Where spinning disk drives don’t really have the performance to ‘push the bandwidth envelope’ within a compute system, SSDs do. Like most performance discussions, identifying where the bottleneck exists is important so that component improvements don’t just move that bottleneck around. Simply replacing spinning disk drives with SSDs can lead to disappointing performance as legacy disk array controllers can’t typically support the dramatic improvement in IOPS that SSDs provide. In the highest performance SSD environments, simply installing a 6Gb initiator and target devices isn’t enough. Care must be taken to make sure the backplane and internal elements in the data path, as well as external elements in the data path, are up to the task.


When looking to integrate SSDs into an existing storage infrastructure the choice of connectivity type can impact the system in performance, cost, flexibility and potential uptime. SATA offers the same 6 Gb per-port bandwidth as SAS but adds the ability to work with SATA or SAS host connections and also enjoys a two-to-one adoption advantage in the marketplace. PCIe SSDs embedded with multiple NAND chips or discrete drives are the newest option, providing the lowest overall latency by bringing the high-speed NAND flash closer to the CPU. However, being bus-connected means that implementation can be more disruptive when installation, maintenance or upgrades require a system reboot. In the final analysis, when considering SSD connectivity options, SATA may provide the best balance of performance, economics and flexibility and make SSDs attractive for more environments.

Eric Slack, Senior Analyst

Micron Technology is a client of Storage Switzerland