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Hard Choices: An Update on Conventional HDD Storage

Original Article Date: 2011-02-26 

Solid State Drives (SSDs) have been getting all the press recently. With orders-of-magnitude improvements in latency and random read-write performance over conventional hard drives, that's not really surprising. Pretty much all hardware experts see that SSDs will eventually become the norm, and that our beloved mechanical hard drives, like the vacuum tube and CRT display, will become museum curiosities.

That day, though, is still somewhat far away, 5-10 years in my opinion. So in the meantime, conventional hard drives will still be a part of our computing lives.

In the last few years there has been no revolutionary developments in this area. Instead what we've seen has been an evolution of mature, proven technology toward higher data density and performance. Most of us are aware of the dramatic increases in available storage on the average 3.5" drive, but perhaps not so many are aware that all market segments of conventional hard drives today are about twice as fast as their equivalents only a few years ago. This change can make a real difference to the overall user experience when it comes to upgrading older hard drives with newer models.

Current Hard Drive Market Segments

When choosing a conventional hard drive today, you're faced with a similar set of market segments from several years ago, except for the addition of SSDs, and the split in the 7,200rpm desktop market into two. These segments are:

  • Solid State Drives (SSDs). Characterized by extremely low latencies and extremely high random IOPS (input/output operations per second), these drives are still very expensive per unit of storage compared with conventional drives, and so at this time only fulfil a speciality segment in the market, such as caches for workstation usage and RAID systems, and for boot/OS drives in high performance gaming rigs. SSDs are extremely variable in performance between brands/models and so research should be done before buying.
  • Serial Attached SCSI (SAS). SCSI was and still is the preserve of high-end enterprise storage, where extra reliability and performance is needed for servers, and where very high performance is desired in workstation scratch volumes (temporary swap files). SAS is simply the serial interface update to SCSI, making drive connections much simpler than before. SAS drives typically have half the failure rate of SATA drives, and are able to increase performance and reduce latency with very high spindle speeds (10krpm and 15krpm). Like SCSI, though, SAS drives also have the premium price tag, and require a special controller to run them, so applications are still limited to the high-end specialist segment.
  • 10,000rpm SATA. Just like a few years ago, there is still only one player in this market, and that's Western Digitial, with their Raptor (now Velociraptor) series. Developed by WD initially for the high end gamer market, these drives have become popular with workstation users who desire the much lower latencies enabled by the high 10krpm spindle speed. Prices still remain high however.
  • 7,200rpm SATA Enterprise/RAID Editions. Targetted specifically at providing better compatibility with hardware RAID controllers, these drives are also usually faster and more reliable than their desktop counterparts, though expect to pay a reasonable premium for that privilege. Examples include the WD "RE" and Seagate "NS" series.
  • 7,200rpm SATA Performance Desktop Editions. A new phenomena in recent years as hard drive manufacturers have sought to differentiate between budget and performance models in the world's largest hard drive market - the 7,200rpm Desktop Drive. At a relatively small premium, significantly higher streaming data rates are possible with these drives compared to ordinary desktops. An example of this segment is the WD "Black" series.
  • SATA Basic Desktop Editions. These drives form the largest volume segment, and are intended for the standard desktop user. Today they represent excellent value for money, and perform very well compared to their predecessors. This market also includes eco-friendly drives that are intended to reduce power consumption through variable spindle speeds (5,400-7,200) and power-off when idle states. Examples include WD's "Green" Series.

Performance By Market Segment

I've assembled below a number of graphs which show an average of performance across the different market and technology segments of hard drives, based upon my own testing of the latest models and third party research.

Graph showing typical latencies in various HDD market segments

This graph shows that conventional drives are no comparison to SSDs when it comes to drive response time (latency), since there are no moving parts involved - response is as fast as a set of transistor switches. Between the conventional drives, however, it can be seen that there is a dividing line between the top-end drives at 10-15kprm spindle speed and the standard 7200rpm drives. Note also the significant difference between the performance and "green" segment of 7200rpm desktops. 

Graph showing typical IOPS (I/O Operations Per Second) in various HDD market segments. Note the logarithmic scale.

IOPS benchmarks, which record the total number of random disk read/write operations within a given time span are considered to provide the best picture of typical disk usage, which is lots of simultaneous random reads and writes, instead of streaming large files. The above graph shows the clear link between latency and IOPS. Those drives that have low latencies show very well - this is the big draw for SSDs, and why the hype about them is justified.

Graph showing typical streaming I/O rates in various HDD market segments

The above graph shows the raw data transfer rates of drives in the various market segments. There is much less here to separate the men from the boys, with the fastest SATA-based SSDs performing only twice as well as the average desktop drive. Note how well, particularly, though, the SAS drives perform. This benchmark is especially important in determining operating system and application load times, and in file transfers.

Graph showing cost per terabyte of storage between the different drive segments

Last, but not least, we bring money into the equation. The above graph shows the remarkable difference in relative cost between each of the market segments, and indicates particularly the high cost of SSDs, the reason why we'll be using conventional drives for years to come.

Electronics Nexus - "Drive Consultants"

A computer system is only as strong as the weakest link. The secret of a good system builder is working out how best to spend the customer's money so that a minimum of bottlenecks occur. It's no good spending 80% of the budget on top-end CPUs, for instance, if you don't provide enough RAM to service them. This applies also to storage, particularly in servers, but also in workstations where the user may need a large swap-file or scratch disk for large temporary uncompressed data.

The above review shows the wide variety of pricing and performance within the conventional hard disk market. So there is a lot of lee-way in deciding how best to spend your budget when it comes the storage part of your next system.

At Electronics Nexus, we have built hundreds of high-performance computer systems, which has given us a unique insight into understanding a customer's needs and translating them into the most cost-efficient best-performing system they can get for their specific purpose. And that alone makes us worth it!

So, what drives will you want in your next system?

Best regards,

Ben Ranson
Chief Systems Engineer
Electronics Nexus