Choosing a SATA Drive: Three Factors to Consider

Posted on December 20, 2013 By Scott Alan Miller


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 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. They 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. They range 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 influence planning decisions in different directions. But, overall, they 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. They are in a price ratio of almost exactly 2:1 making it favorable, for equal capacity, 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.

Photo courtesy of Shutterstock.

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