I choose to categorize Western Digital’s SATA drive lineup for several reasons. One is that WD is the current market leader in spinning hard drives so this makes the categorization most useful to the greatest number of people, the “color coded” line is, based on anecdotal evidence, far and away the chosen drive family of the small business market where the diagnosis is most important and SATA drives retain the most disparity of features and factors making them far more necessary to understand well. While technically the only difference between a SAS (SCSI) and SATA (ATA) drive or even a Fibre Channel (FC) drive is nothing but the communications protocol used to communicate with them, in practical terms SAS and FC drives are only made in certain, high reliability configurations and do not require the same degree of scrutiny and do not carry the same extreme risks as SATA drives. Understanding SATA drive offerings is the more important for practical, real world storage needs.

WD has made understanding their SATA drive line up especially easy by adding color codes to the majority of their SATA drive offerings – those deemed to be “consumer” drives, and an “E” designation on their enterprise SATA drives and one outlier, the high performance Velociraptor drives which seek to compete with common SAS performance for SATA controllers. Altogether they have seven SATA drive families to consider covering the gamut of drive factors. While this diagnosis will apply to the easy to understand WD lineup, by comparing factors here with the offerings of other drive makers the use cases of their drives can be determined as well.

In considering SATA drives, three really key factors stand out as being the most crucial to consider (outside of price, of course.)

URE Rate: URE, or Unrecoverable Read Error, is an event that happens, with some regularity, to electromechanical disk storage media where a single sector is unable to be retrieved. In a standalone drive this happens from time to time but generally only affects a single file and users typically see this as a lost file (often one they do not notice) or possible a corrupt filesystem which may or may not easily be corrected. In healthy RAID arrays (other than RAID 0), the RAID system provides mirroring and/or parity that can cover for this sector failure and recreate the data protecting us from URE issues. When a RAID array is in a degraded state UREs are a potential risk again. In its worst case, a URE on a degraded parity array can, in some cases, cause total loss of an array (all data is lost.) So considering UREs and their implications in any drive purchases is extremely important and is the primary driver of cost differential in drives of varying types. URE varies from the low end at 10^14 to the high end at 10^16. The numbers are so large that they are always written in scientific notation. I will not go into an in-depth explanation of URE rates, ramifications and mitigation strategies here, but understanding URE is critical to decision making around drive purchases, especially in the large capacity, lower reliability space of SATA drives.

Spindle Speed: This is one of the biggest factors to most users, spindle speed directly correlates to IOPS and throughput. While measurements of drive speed are dynamic, at best, spindle speed is the best overall way to compare two otherwise identical drives under identical load. A 15,000 RPM drive will deliver almost exactly double the IOPS and throughput of a 7,200 RPM drive, for example. SATA drives commonly come in 5,400 RPM and 7,200 RPM varieties with rare high performance drives available at 10,000 RPMs.

Error Recovery Control (ERC): Also known as TLER (Time Limited Error Recovery) in WD parlance, ERC is a feature of a drive’s firmware which allows for configurable time limits for read or write errors which can be important when a hard drive is used in a RAID array as often error recovery needs to be handled at the array, rather than the drive, level. Without ERC, a drive is more likely to be incorrectly marked as failed when it has not. This is most dangerous in hardware based parity RAID arrays and has differing levels of effectiveness based on individual RAID controller parameters. It is an important feature for drives assumed for use in RAID arrays.

In addition to these key factors, WD lists many others for their drives such as cache size, number of processors, mean time between failures, etc. These tend to be far less important, especially MTBF and other reliability numbers as these can be skewed or misinterpreted easily and rarely offer the insight into drive reliability that we expect or hope. Cache size is not very significant for RAID arrays as they need to be disabled for reasons of data integrity. So outside of desktop use scenarios, the size of a hard drive’s cache is generally considered irrelevant. CPU count could also be misleading as single CPUs may be more powerful than dual CPUs if the CPUs are not identical and the efficacy of the second CPU is unknown. But WD lists this as a prominent feature of some drives and it is assumed that there is measurable performance gain, most likely in latency reduction, through the addition of the second CPU. I do, however, continue to treat this as a trivial factor and mostly only useful as a point of interest rather than as a decision factor
The drives.

All color-coded drives (Blue, Green, Red and Black) share one common factor – they have the “consumer” URE rating of 10^14. Consumer is a poor description here but is, more or less, industry standard. A better description is “desktop class” or suitable for non-parity RAID uses. The only truly poor application of 10^14 URE drives is in parity RAID arrays and even there, they can have their place if properly understood.

Blue: WD Blue drives are the effective baseline model for the SATA lineup. They spin at the “default” 7,200 RPMs, lack ERC/TLER and have a single processor. Drive cache varies between 16MB, 32MB and 64MB depending on the specific model. Blue drives are targeted at traditional desktop usage – as single drives with moderate speed characteristics, not well suited to server or RAID usage. Blue drives are what is “expected” to be found in off the shelf desktops. Blue drives have widely lost popularity and are often not available in larger sizes. Black and Green drives have mostly replaced the use of Blue drives, at least in larger capacity scenarios.

Black: WD Black drives are a small upgrade to the Blue drives changing nothing except to upgrade from one to two processors to slightly improve performance while not being quite as cost effective. Like the Blue drives they lack ERC/TLER and spin at 7,200 RPM. All Black drives have the 64MB cache. As with the Blue drives, Black drives are most suitable for traditional desktop applications where drives are stand alone.

Green: WD Green drives, as their name nominally implies, are designed for low power consumption applications. They are most similar to Blue drives but spin at a slower 5,400 RPMs which requires less power and generates less heat. Green drives, like Blue and Black, are designed for standalone use primarily in desktops that need less drive performance than is expected in an average desktop. Green drives have proven to be very popular due to their low cost of acquisition and operation. It is assumed, as well, that Green drives are more reliable than their faster spinning counterparts due to the lower wear and tear of the slower spindles although I am not aware of any study to this effect.

Red: WD Red drives are unique in the “color coded” WD drive line up in that they offer ERC/TLER and are designed for use in small “home use” server RAID arrays and storage devices (such as NAS and SAN.) Under the hood the WD Red drives are WD Green drives, all specifications are the same including the 5,400 RPM spindle speed, but with TLER enabled in the firmware. Physically they are the same drives. WD officially recommends Red drives only for consumer applications but Red drives, due to their lower power consumption and TLER, have proven to be extremely popular in large RAID arrays, especially when used for archiving. Red drives, having URE 10^14, are dangerous to use in parity RAID arrays but are excellent for mirrored RAID arrays and truly shine at archival and similar storage needs where large capacity and low operational costs are key and storage performance is not very important.

Outside of the color coded drives, WD has three SATA drive families which are all considered enterprise. What these drives share in common is that their URE rate is much higher than that of the “consumer” color coded drives. Ranging from URE 10^15 to 10^16 depending on model. The most important result of this URE rate is that these drives are far more applicable to use in parity RAID arrays (e.g. RAID 6.)

SE: SE drives are WD’s entry level enterprise SATA drives with URE 10^15 rates and 7,200 RPM spindle speeds. They have dual processors and a 64MB cache. Most importantly, SE drives have ERC/TLER enabled. SE drives are ideal for enterprise RAID arrays both mirrored and parity.

RE: RE drives are WD’s high end standard enterprise SATA drives with all specifications being identical to the SE drives but with the even better URE 10^16 rate. RE drives are the star players in WD’s RAID drive strategy being perfect for extremely large capacity arrays even when used in parity arrays. RE drives are available in both SATA and SAS configurations but with the same drive mechanics.

Velociraptor: WD’s Velociraptor is a bit of an odd member of the SATA category. With URE 10^16 and a 10,000 RPM spindle speed the Velociraptor is both highly reliable and very fast for a SATA drive competing with common, mainline SAS drives. Surprisingly, the Velociraptor has only a single processor and even more surprisingly, it lacks ERC/TLER making it questionable for use in RAID arrays. Lacking ERC, use in RAID can be considered on an implementation by implementation basis depending on how the RAID system interacts with the drive’s timing. With the excellent URE rating, Velociraptor would be an excellent choice for large, higher performance parity RAID arrays but only if the array handles the error timing in a graceful way, otherwise the risk of the array marking the drive as having failed is unacceptably high for an array as costly as this would be. It should be noted that Velociraptor drives do not come in capacities comparable to the other SATA drive offerings – they are much smaller.

Of course the final comparison that one needs to make is in price. When considering drive purchases, especially where large RAID arrays are concerned or for other bulk storage needs, the per drive cost is often a major, if not the driving, factor. The use of slower, less reliable drives in a more reliable RAID level (such as Red drives in RAID 10) versus faster, more reliable drives in a less reliable RAID level (such as RE drives in RAID 6) often provides a better blend of reliability, performance, capacity and cost. Real world drive prices play a significant factor in these decisions. These prices, unlike the drive specifications, can fluctuate from day to day and swing planning decisions in different directions but, overall, tend to remain relatively stable in comparison to one another.

At the time of this article, at the end of 2013, a quick survey of prices of 3TB drives from WD give these approximate breakdown:

Green $120
Red $135
Black $155
SE $204
RE $265

As can be seen, the jump in price primarily comes between the consumer or desktop class drives and the enterprise drives with their better URE rates with Red and RE drives, both with ERC/TLER, being in a price ratio of almost exactly 2:1 making, for equal capacity, it favorable to choose many more Red drives in RAID 10 than fewer RE drives in RAID 6, as an example. So comparing a number of factors, along with current real world prices, is crucial to making many buying decisions.

Newer drives, just being released, are starting to see reductions in onboard drive cache for exactly the reasons we stated above, drives designed around RAID use have little or no purpose to having onboard cache as it needs to be disabled for data integrity purposes.

Drive makers today are offering a wide variety of traditional spindle-based drive options to fit many different needs. Understanding these can lead to better reliability and more cost effective purchasing and will extend the usefulness of traditional drive technologies into the coming years.