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Seagate Cheetah X15 ST318451LW
  June 1, 2000 Author: Eugene Ra  
Evaluation unit provided by Seagate Technology.


When it comes to increasing spindle speed, Seagate Technology has undoubtedly been the pioneer. Announced over ten years ago, the company's original Elite was the first to reach a then-scorching 5400 rpm spindle speed. Four years later (1993), Seagate introduced the Barracuda, the first hard disk to feature 7200 rpm operation. Most recently, in late 1996, the manufacturer introduced the Cheetah, the first drive to feature 10k rpm speeds. Two years ago, Hitachi Ltd caught the industry's attention with its Pegasus, a drive featuring a 12k rpm spindle speed. As an incremental upgrade, however, Hitachi's initiative never quite caught on... and 10k operation still anchored the high-end. Finally, however, in the year 2000, it has been left once again to Seagate to ascend towards the next level of performance. Enter the Cheetah X15.

The X15 is the first hard drive to feature a 15,000 rpm spindle speed. Unlike Hitachi's attempt at 12k operation, 15k speeds are a significant improvement, along the lines of going from 5400 rpm to 7200 rpm or from 7200 to 10k speeds. With other manufacturers gearing up for eventual migration, 15k seems here to stay.

What exactly do spindle speed increases deliver? As many veteran readers are surely aware, sending and retrieving data to and from the hard disk consists of two primary stages:

  • Getting the read/write heads into position to read or write the data ("Positioning," as Charles Kozierok of the PC Guide calls it)
  • Reading the data from or writing the data to the platter ("Transfer")
While there are some notable exceptions, in this day and age, taking into account the current relative speeds of hard disk mechanisms and the disk access patterns of contemporary operating systems, the former (positioning) is clearly the dominant bottleneck in the vast majority of applications. It is thus of paramount importance to minimize the time spent positioning the heads. Such reductions in many cases will result in directly proportional increases in overall hard disk speeds.

One can further divide the act of "positioning" into two discrete sections:

  • Moving the actuator (on which the heads are mounted) into place over the correct track
  • Waiting for the correct sectors (where the desired data is to be read from or written to) to rotate under the heads
The former is commonly known as "seek time," the later, "rotational latency." The sum of the two figures is "access time." As we move from generation to generation, at least in the SCSI hard disk domain, we've been blessed with a downward trend in seek times. Witness, for example, the evolution of the 10k rpm drive. Seagate introduced the first-generation Cheetah 4LP with a seek time of 7.7 milliseconds. The second-generation Cheetah 9LP featured a seek time of 5.2 milliseconds. The third-generation-class IBM Ultrastar 18LZX shaved its average seek time to 4.9 milliseconds. Finally, the fourth-generation equivalent Quantum Atlas 10k II has reached a svelte 4.7ms seek.

However, each of the four drives, whether we're talking the 1997 Cheetah 4LP or the 2000 Quantum Atlas 10k II, features a rotational latency of 3 milliseconds. This is, of course, due to the models' consistent spindle speeds. Rotational latency, after all, is a direct derivative of spindle speed. 10000 rotations per minute translates into 6 milliseconds per rotation. The best-case latency scenario would involve the required sectors (either containing the data to be read or the space for data to be written to) passing under the heads as soon as the actuator arrives at the proper track. Conversely, a worst case scenario would have the same sectors just past the heads, requiring a full 6 millisecond rotation before the correct sectors re-arrived under the heads. The average case would be the average between the best and worst cases... 3ms.

Decreases in rotational latency are therefore much more rare, occurring only when spindle speeds are increased. A 15k spindle speed results in the first significant improvement we've seen to rotational latency in four years, yielding a figure of 2 milliseconds. Combining this with the average seek time in today's top drives of 5 milliseconds creates an average access time of 7 milliseconds. When access times are this low, a 1 millisecond improvement is significant... it results in 14% faster accesses.

Of course, Seagate would never let a revolutionary (no pun intended) spindle speed increase to go unaccompanied by anything less than a state-of-the-art seek time. The manufacturer has thus reduced the platter size of the Cheetah X15 to about 2.6 inches, down from the 10k rpm standard of 3.0 inches (which in itself is a reduction in size when compared to the 3.5" platters found in most 7200rpm and 5400rpm drives). As a result, the X15 sports a seek time of just 3.9 milliseconds. Take this seek time and combine it with the drive's low rotational latency and the result is an access time of 6 milliseconds... an improvement of 30% or so over current 10k rpm models!

Some skeptics may wonder why Seagate chose to further reduce platter size to achieve its decrease in seek time rather than applying more power to the actuator and creating faster seeks. After all, larger platters would result in the dual benefit of larger capacities and faster sequential transfer rates.

Through interviews with many of its top customers, the company found that its drives were being used with partitions that spanned only one-half or even one-third of total capacity. The reason? Clients wanted to improve on drive seek times by restricting seeks to only a fraction of the platter's span.

Combine this with Seagate's desire to have the drive integrate into all situations (noise and heat wise) where the third-generation Cheetah 18LP worked. All other things being equal, a higher spindle speed will require a more powerful motor, which will result in both more noise and more heat. Smaller platters, however, mean less mass that must be spun at the higher rate, thus evening things out quite a bit. Summarized:

Smaller Platters
Advantages Disadvantages
  • Lower Seek Times
  • Lower Noise Levels
  • Lower Heat Levels
  • Lower Transfer Rates
  • Lower Capacities

Let's take a moment to examine the sequential transfer rate (STR) situation. As we've stated before, only a small handful of applications are bottlenecked by STR. The vast majority are access time dependent. Secondly, the smaller sector-per-track count (one of two major factors in determining STR) in the outer zones of the reduced diameter platters in the X15 is countered by the drive's 15k rpm operation (the other major factor). Finally, keep in mind that the minimum STR, that of the innermost zone, should be well above what we've seen in the past. After all, the X15's inner zone benefits from a linear data density that is state-of-the-art combined with the higher spindle speed... i.e., both the benefits and none of the disadvantages.

The Ultra160/m Cheetah X15 is available in just one size. Its five platters, each storing 3.7 gigs of data, total up for 18.4 gigs of capacity. No, not huge by today's standards, but certainly enough capacity to satisfy the drive's target market: e-commerce servers, transaction processing, data mining, etc. Not to mention rip-roaring power users. After all, no power user in his right mind would have the OS, apps, data, and swapfile all on the same drive, right?

A 4 megabyte buffer rounds out the package. As is standard for Seagate's offerings, an "A/V Ready" version equipped with a 16 meg buffer will also be available. The drive features a standard five-year warranty. Despite the novelty of its design, Seagate maintains through its MTBF figures that the X15 is every bit as reliable as the 10k rpm workhorses that preceded it.

Now then, 'tis the moment that we've been waiting for: How does the Cheetah X15 perform? Come with us as we examine!

WB99/Win2k Low-Level Measurements

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Click here to examine the STR graph for this drive

The Cheetah X15 turns in an average access time of 6.8 milliseconds... considering the drive's specified 3.9 millisecond seek time and its 2 millisecond rotational latency, this is actually a tad on the high side, even considering various overheads. Take a look, for example, at the Quantum Atlas 10k II, the previous access time champion. It meets its expected access time dead on, at 7.8 milliseconds. Yes, the X15 has the lowest measured time to date, but not nearly by the margin we expected.

Despite its smaller sector-per-track count in its outermost zone, the X15 turns in record sequential transfer rates. WinBench 99 measures the drive at 41 MB/sec, just a smidgeon ahead of the Atlas 10K II's 40 MB/sec score. The drive's inner zone transfer rate, relatively speaking, is even more impressive, weighing in at nearly 29 MB/sec, almost a full 4 MB/sec faster than the Atlas 10k II or the Cheetah 18XL, for that matter.

WB99/Win2k WinMarks

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Here are some results that'll undoubtedly surprise many SR readers: The Cheetah X15 does not dethrone the Atlas 10k II in the Business and High-End Disk WinMarks. The Quantum in the Business Disk WinMark edges out the Seagate by an insignificant 1%. In the High-End WinMark, however, we find the X15 lagging behind the Atlas by a more substantial 9%.

This may be further indication of something that we've alluded to before... that WB99's tests are becoming less and less reliable as drives advance and as the test itself ages. In particular, it seems that optimizing firmware for the benchmark (unfortunately without corresponding increases in the applications that the benchmark represents) can easily erase even huge access time advantages. Witness for example a comparison between the IBM Deskstar 75GXP and the X15 (not displayed here, but easily configured using the versatile Database!).

The X15 nearly halves the 75GXP's access time yet edges the 75GXP out by only 6% in the Business Disk WinMark. Heck, even the 16% margin that the Cheetah enjoys in the High-End WinMark seems a bit suspect. At any rate, it's some food for thought for those who still swear by WB99. At one time, WB99 was among the best overall measures of HD performance. Unfortunately, it's aged ungracefully.

Let's examine the situation in the SR benchmark of choice, IOMeter!

IOMeter Performance

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IOMeter paints a much different picture than WinBench 99. Here, in Workstation usage, rather than folding to the Atlas 10k II, the X15 destroys it! Under a heavy load, the X15 cruises past the Atlas 10k by a margin of 33%. Figures under some lighter loads are even more impressive. Examine, for example, the difference between the two drives when it comes to light loads. In such cases we have the Seagate leading the Quantum by an amazing 45%! Similar margins apply to the two competitors in File Server scenarios- the X15 leads by large margins.

The comparison between the X15 and the IBM Deskstar 75GXP also pans out more along expected lines. In IOMeter, the Cheetah leads the Deskstar by margins up to a whopping 69% (see the Database for a comparison!). IOMeter certainly paints quite a different picture than WB99.


This brings us to heat and noise, certainly two very important factors when examining the first of a new breed of drives. As we've stated earlier, the Cheetah X15 combines a higher spindle speed with lighter platters in hopes of maintaining the same heat and noise levels as its predecessor. When it comes to heat, Seagate has succeeded. This means, of course, that the drive operates quite warmly but not hot to the touch without active cooling in a spacious case. If the intended setup is a bit more cramped, active cooling should be implemented. For a drive that seeks at a mere 4 milliseconds, the X15 is amazingly quiet. Seeks are just a tad louder than the Cheetah 18XL, itself a relatively quiet drive. Unfortunately, though, idle noise has suffered. The drive features a tell-tale high-spindle-speed whine. We must admit we've grown a bit spoiled in this age of third and fourth-generation 10k drives, where high-pitch squeals have pretty much faded away. The X15 features idle noise that we haven't heard of since the likes of the Cheetah 9LP.

In conclusion, the Cheetah X15 is a groundbreaking product that allows industry-giant Seagate to show the market who's truly in charge. The drive will deliver performance that's light-years ahead of the competition. We have only two small caveats. The first is the high-pitch idle whine that the drive exhibits. Since the drive is targeted towards servers that won't have individual users sitting close to the drive, however, this is a minor weakness at worst. Our second is with the drive's 18 gig capacity. There may be those that desire the X15's blazing speed with state of the art capacities... unfortunately, no mix of the two will be available. Complaints aside, the X15 is the drive to get for those who can live with a max of 18 gigs of storage on one drive who need the utmost in speed. There's no other drive like it.

Over three months ago, in our review of the Quantum Atlas 10k II (published the day after Seagate's official X15 announcement), we pointed out (perhaps erroneously) that though the X15 would undoubtedly be faster, its availability would follow well after that of the Atlas. At the time of this writing, due to various delays, the Atlas 10k II has yet to ship to distributors. Meanwhile, Seagate maintains an "early third quarter" date for general availability of the X15. As a result, the gap of time between the two drives may be quite a bit narrower than we anticipated. What interesting times.

Seagate Cheetah X15 ST318451LW
Estimated Price: TBA (<$1000)
Also Available: None
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