The Cheetah X15 is the first disk drive to rotate at 15,000 rpm.
By Jack Fegreus
While many disk drive manufacturers are eyeing the home market for smart devices to open new markets and bolster revenues, Seagate is targeting a very different segment of the storage market with its new Cheetah X15 drive. The 18.2GB X15 drive is primarily designed for use in high-end servers running data-intensive applications such as e-commerce, data warehousing, data mining, professional video, and medical imaging. In addition, the drive is also a strong candidate for use in high- performance workstations involved in multimedia content creation.
The Cheetah X15 is the first commercially available drive to operate at 15,000rpm. By spinning the platters at this velocity, Seagate has lowered the rotational latency of the drive to just 2.0ms. In tandem with this breakthrough rotational speed, Seagate has enhanced the actuator motor of the X15 to deliver an average seek time of just 3.9ms. This pegs average access time down to just 5.9ms.
We conducted all of our tests on a Dell PowerEdge 2400 server, which is configured with a 64bit, 33MHz PCI bus. We tested four Ultra160 SCSI Cheetah X15 drives with a capacity of 18.4GB (ST318451LC) using a QLogic QLA12160 host bus adapter. All tests were run using the Nova Technica benchmark suite under Windows 2000 Advanced Server.
We compared all of our results to earlier tests of the 10,000rpm Ultra160 SCSI Cheetah 18XL (ST318404LC). Like the 18XL drives, all the Cheetah X15 drives that we tested had 4MB data caches. For video editing and other high-end multimedia editing applications, the Cheetah X15 model ST318451LCV drive has a 16MB cache.
Simply by spinning 50% faster, we expected a significant increase in sequential throughput, barring an equal reduction in the linear bit density-measured in bits per inch (bpi)-of the drive's platters. Indeed, a significant increase in streaming performance was measured.
The average access time for the X15 drive was 4.5ms (which includes system caching), compared to 5.5ms for an 18XL drive with a queue depth of 1, which represents an 18% faster response time. The maximum sustainable I/O for both drives peaked at a queue depth of 20. With 20 Daemon processes, the average response time was 41ms on the X15 drive while processing 464 I/Os per second. Under similar conditions, the average response time of the 18XL drive was 51.5ms while processing 351 I/Os per second.
Starting with 8KB data transfers, which is typical for many applications, the Cheetah X15 demonstrated 10% faster throughput compared to the 18XL drives. This advantage increased to 20% for 64KB data transfers, which are used by system utilities such as backup. In our tests of a single drive, streaming throughput peaked at around 40GBps.
For a logical volume built using four physical drives, the performance gain under Windows 2000 was not quite as good. For a striped volume (RAID 0) the performance gain was about 10%, and peaked around 135MBps. For a RAID-5 volume, the performance improvement was statistically insignificant. Once again, read performance for RAID 5 exhibited a significant performance penalty on the order of 35% to 40%.
For most high-end server applications, however, the most important metric is the number of I/O operations that can be performed in a given amount of time.
The Cheetah 18XL introduced a number of performance design features, which are further refined in the Cheetah X15, to enhance performance in a high-transaction environment.
When pushing the rotational speed of drives-first with the 18XL and later with the X15-to lower rotational latency, Seagate faced a big problem. Rotating the disk platters at higher speeds sig nificantly increases power consumption, as well as the amount of heat to dissipate. To solve this problem, Seagate lowered the mass of the media.
Seagate uses five platters with smaller diameters in the 18GB X15 drive, compared to three platters and six heads in the 18GB 18XL drive. The net performance result for these design changes was a significant boost in data-access characteristics in a typical transaction-processing environment.
The X15 drive's performance advantage scaled well when four physical drives were configured in a logical RAID-0 stripe set. Peak I/O processing on a four-drive stripe set was measured at 2,058 I/Os per second using X15 drives, and 1,301 I/Os per second using 18XL drives. This represents a 58% gain in the maximum load of I/Os per second handled.
Using the Nova Technica load benchmark, we measured an average access time of 4.5ms for an X15 drive, compared to 5.5ms for an 18XL drive with a queue depth of 1. Caching by the operating system accounts for the fact that this is faster than the drive's physical spec. This translated into a load of 222 I/Os per second (1KB to 16KB reads in a normal distribution centered on 8KB) for the X15 drive, compared to 182 I/Os per second for the 18XL. This represents an 18% faster response time and a 22% increase in base I/O loading.
The maximum sustainable I/O for both drives peaked at a queue depth of 20. With 20 Daemon processes, we measured an average response time of 41ms on the X15 drive while processing 464 I/Os per second. Under similar conditions, we measured an average response time of 51.5ms on the 18XL drive while processing 351 I/Os per second. As in the base load case, under peak I/O loading we again measured a 20% faster response rate on the X15 drive and processed 32% more I/O requests per second.
The X15 drive's performance advantage scaled well when four physical drives were configured in a logical RAID-0 stripe set. Peak I/O processing on a four-drive stripe set was measured at 2,058 I/Os per second using X15 drives, and 1,301 I/Os per second using 18XL drives. This represents a 58% gain in the maximum load of I/Os per second handled. Once again, we measure a 20% performance penalty for reads with a RAID-5 logical volume.
More importantly, with all of the improvements in performance, the Seagate X15 drive has the same power consumption and cooling requirements as the earlier generation Cheetah 18LP, which operates at 10,000rpm. As a result, OEMs and system integrators can use this new class of drive in existing platforms and applications without having compatibility issues or requiring infrastructure changes.