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Seagate Cheetah 18XL ST318404LW
  April 25, 2000 Author: Eugene Ra  
Evaluation unit provided by Adaptec Corp.
See also: Seagate Cheetah 36LP ST336704LW


Ever since the Cheetah's inception, Seagate has been careful to distinguish between families of its top-of-the-line drive. The original Cheetah 4LP, for example, was released simultaneously with the Cheetah 9, a drive that packed twice the capacity by doubling its platter count and thus shipping in a larger (1.6" high) package. Similarly, the second-generation Cheetah 9LP, consisting of 4.5 GB and 9.1 GB models, shipped along with the 1.6" Seagate Cheetah 18. The third-generation brought us the Cheetah 18LP, available in 9.1 and 18.2 gig capacities, with the Cheetah 36.

Each generation delivered us drives with a 1" form factor that performed similarly along with a 1.6" high drive designed with massive capacity in mind. The high capacity models featured a few drawbacks. The first, obviously, was the increased physical size. Such large form factors are more difficult to integrate... in terms of space, of course, but also cooling concerns. The second drawback was a slight performance hit: the increased platter count resulted in heavier actuator arms that couldn't access data quite as fast as its smaller, more nimble relatives.

In some ways, Seagate's fourth-generation Cheetahs mirror the strategy of the past. The flagship low-profile model, the Cheetah 36LP, is a drive featuring 36 gigs of capacity. The Cheetah 73, on the other hand, sacrifices small size for massive capacity. When it comes to the former, there's a unit featuring 36 gigs of capacity... and... that's it. Interestingly, there are no 18.2 or 9.1 GB Cheetah 36LPs. Such capacities have instead been given the fourth-generation moniker of "Cheetah 18XL." Why the difference?

Differences between the 18XL and the 36LP aren't discernable through the specs. Both drives feature 10,000rpm operation. Both units pack 6.1 gigs of data per platter. They both have a four megabyte buffer. Finally, each is rated with a seek time at 5.2 milliseconds.

Our contact at Seagate, however, is convinced that a measurable difference exists between the two lines. Apparently, in typical drive family situations, lower capacity drives that feature a correspondingly lower platter count don't necessarily feature lighter actuator arms. For instance, the 9 gigabyte Cheetah 18LP, despite featuring only three disks, is equipped with the same actuator assembly as the larger 18 gig model- one that features 12 arms. Thus, there really isn't any savings when it comes to smaller units. This is the principal reason that, contrary to popular belief, smaller capacity drives within the same family do not perform any better (in fact, they lag very slightly) than the flagship unit.

The Cheetah 18XL and 36LP, however, feature physically different actuator assemblies. The 36LP, featuring 12 platter-sides of data, is equipped with 12 arms. The 18XL, sporting only 6 platter-sides, comes with a correspondingly reduced assembly, featuring only 6 arms. Thus, despite the identical specified seek times between the two drives, the 18XL in theory should seek faster due to its lighter assembly. Does this pan out? Read on!

The Cheetah 18XL marks the first drive to be benchmarked solely in our new testbed. Our older system, finally, has moved on to not-necessarily-greater, not-necessarily-better things (its going to become my personal Linux experimentation box). It is our hope that the twenty-four ATA and SCSI drives that have been tested in both units provide sufficient overlap to interpolate the increasingly-irrelevant scores that a current drive may have posted on our older system. Now then, let's move on to some figures!

WB99/Win2k Low-Level Measurements

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Click here to examine the STR graph for this drive

When it comes to sequential transfer rates, the Cheetah 18XL and Cheetah 36LP post virtually identical results. Interestingly, however, the 18XL features a negligible but not insignificantly faster transfer rate in its innermost zone, sliding by the 36LP by a 4% margin.

It's seek time that we're most interested in, however. Here the 18XL's optimized, reduced-weight actuator assembly allows it to power past the 36LP with an access time a whole... one-tenth of one millisecond less. Yes, that's right, the 18XL posts an average access time of 8.9 milliseconds compared to the 36LP's 9.0 milliseconds. Hmm... a 1% margin wasn't exactly what we expected. Are other differences between the two just as negligible? Let's find out!

WB99/Win2k WinMarks

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Despite the small differences in low-level marks, the gain posted by the Cheetah 18XL over the 36LP in WinBench 99's Business Disk WinMark is substantial. The smaller drive cruises by its larger brother by a margin of 14%. The difference in the High-End test shrinks somewhat, however. Here the 18XL comes out on top by 7%.

Many readers are likely interested in comparisons between the Cheetah 18XL and's current leaderboard champ, the Quantum Atlas 10k II. Despite its gains over the 36LP, the 18XL simply can't keep up with the Quantum drive when it comes to WinBench 99. In such a comparison, the 18XL falls behind the Atlas by margins of 15% and 25% respectively in the Business and High-End Disk WinMarks.

However, everyone knows that these days, IOMeter is where the action is here at Let's see how the 18XL stacks up in this more important test.

IOMeter Performance

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Here's where things get interesting. As was the case in WinBench 99, the Cheetah 18XL produces measurable gains when compared to the 36LP despite their similar access times. In an access pattern that simulates workstation usage, the 18XL slides past the 36LP by 6% under a linear load. The difference becomes greater as load increases... all the way up to an 11% advantage under heavy loads.

How does the Cheetah 18XL, however, compare to the Atlas 10k II? We've got some very intriguing results here. The Atlas' superior seek time grants it a small advantage over the 18XL when it comes to a linear workstation scenario. In all other cases, however, the Cheetah 18XL bests the Atlas 10k II! The difference under a heavy load, for example, is 7.5%. The Cheetah likes lighter loads best, however, where we find it leading the Quantum drive by a substantial 12%. When it comes to workstation usage, the 18XL is the faster drive.

It's important to note that in other scenarios, such as File Server or Database access patterns, the race is much closer. In such usage, the Atlas 10k II manages to hold on to its lead in Linear, Medium, and Heavy loads. The Cheetah 18XL, however, sneaks past in Very Light and Light situations.

We invite readers to peruse the Database for a full breakdown of IOMeter differences.


The Cheetah 18XL's lower platter count and lighter actuator assembly results in noticeable benefits when it comes to heat and noise. The drive's seeks are a bit more muffled than those on the Cheetah 36LP. It still isn't, however, whisper-quiet like many of today's ATA drives. The drive operates relatively coolly for a 10k unit. Used in a well-ventilated case with decent space (say, an empty drive bay above the 18XL), the drive becomes warm but not hot to the touch.

-- The Safe Buy Award --

Simply put, this designation means we'd purchase this product without regret. Sure, there may be a slightly better, slightly faster, and/or slightly less-expensive model from a competitor, but you can't go wrong with this particular unit. This award is applicable, of course, to all units at the top of their class, but also applies to units that, though not quite best-of-class, provide a strong showing nonetheless.In conclusion, the Cheetah 18XL does indeed prove to be a faster performer than the same-generation Cheetah 36LP. Kudos to Seagate for choosing to go with a separate name! More Kudos to Seagate for actually producing a drive that once again makes the venerable manufacturer a contender in the "fastest drive" stakes. Those needing a large-capacity drive or a drive that excels in file server or database usage would still be wise to look at the Quantum Atlas 10k II. If you don't need more than 18 gigs of capacity on a single drive, the Cheetah 18XL is the drive of choice when it comes to workstation usage.

Seagate Cheetah 18XL ST318404LW
Estimated Price: $549
Also Available: ST39204LW (9.1 GB)
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