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Points to remember Choosing between SSD and SAS Drives

With the ever changing technology and innovations, a  variety of storage devices have been introduced into the market. This creates a lots of confusion about what types of drives to use for what data types. Adding to the confusion is Serial ATA (SATA) and SAS, which refer to disk drive interfaces, and Solid State Drive (SSD) which refers to a particular kind of internal technology. Then there are considerations of random access performance, sequential performance, cost, density and reliability.

All these factors make selecting the right drives a challenge. This article offers six tips for navigating through this complexity to help the client pick the right solutions for his needs.

1. Interface Type and Disk Performance:

In the past, SAS and SATA were used as convenient shorthand for fast (SAS) or dense (SATA) disk drives. Now, however, we have SSD drives with SATA interfaces as well as inexpensive and dense but relatively slow-IOPS 7200 RPM drives with SAS or even Fiber Channel interfaces. Users can no longer make blanket assumptions like “SAS is better for databases.” For example,it would not be wise to compare an ultra fast SLC SSD with a SATA interface with a relatively sluggish 7200 RPM NL-SAS drive.

Considering the fact that for a 40TB system, users would need 40 of the 1TB SFF NL-SAS drives, while only needing 10 of the 4TB drives referenced above – one fourth as many. Also, because the 4TB drive much more reliable, there would be 5 times as many SFF drives failing per year. Additionally, the 4TB drive would only consume 113 Watts, whereas the SFF drives would consume over 200 Watts for the same capacity.

2. For Best Price to GB, 3.5-inch RPM SATA Is recommended:

Web Hosting companies have a seemingly endless variety of pricing models, but one constant aspect is that the 2.5-inch systems cost twice as much per gigabyte as 3.5-inch systems, assuming both are using “enterprise-grade” drives that are quality tested. But as previously noted, the 3.5-inch solution will be far more reliable.

3. HDD Performance is mostly about Density and Mechanical Speed:

The random access or transactional (IOPS) performance of spinning drives is dominated by the access time, which in turn is determined by the rotational latency and seeking time. Interface performance has little to no influence on IOPS, except in the negative sense that complex or new interfaces sometimes have incorrect  or immature drivers which can hurt IOPS. Highly random applications which benefit from high IOPS drives include email servers, databases environments.

Sequential performance, which is important for applications like video and D2D backups, are dominated by the RPM of the drive multiplied by the bits per cylinder. This number decreases 50 percent or more as the drive moves from the outermost to the innermost cylinders. Also, as long as the interface is fast enough to keep up as is the case in all modern hard drives, the interface speed and the quantity of interface ports has no major effect on sustained performance. The fastest drives today can sustain less than 200 MB/s, which is less than the performance of a single 3 GB SATA port.

6. Quality control:

The user needs to know how the vendor treats the subject of hard drives. Of late, a number of hosting companies are moving to a logistics model where the drives are not tested in the storage array until it arrives at the customer site. Others are phasing out the rigorous qualification and ongoing screening process that once was commonplace. Some no longer perform specific qualification checks between drive hardware and firmware revisions, and the hardware and firmware revisions of all the components of the storage array. This is a recipe for disaster, especially with large numbers of drives at a site.

The user would want to make sure the disk drives you are getting are “enterprise-class,” not consumer grade. Enterprise-class disk drives are those that pass the manufacturer’s highest quality and reliability tests. Frequently new drive technology appears in consumer products before they are released in storage systems designed for data centers.

There are many considerations when selecting the right drive type for the user's environment. These few tips can help you sort through some of the confusion created by the market hype.The user needs to be clear on his requirements and on the type of drives,user interfaces, price/GB, and finally, reliability of the drivers.