Enhancements to the Ultrium 2 specification over Ultrium 1 can be found in many of the same areas that Quantum made to SDLT 320 over SDLT 220: track density and tape speed. Using the same media as Ultrium 1, Ultrium 2 lays down 512 data tracks instead of 384 tracks for a 33% increase in track density. This allows an Ultrium 2 drive to lay down 7.32kb of data on a millimeter of tape.

The adaptive tape speed (ATS) range on the Ultrium 2 has been increased across the board, from the minimum to the peak streaming tape speed, by about 40%. All Ultrium drives use an ATS algorithm that monitors the host’s data rate to vary the tape speed in order to keep the drive’s data buffer full or empty (depending on whether a restore or a backup is in process) and thereby minimize any mechanical stops and starts to reposition the tape. At the same time, the main buffer on the HP StorageWorks Ultrium 460 tape drive has been increased to 64Mbytes from the 16MB used in the earlier HP StorageWorks Ultrium 230.

Given just these changes, the areal bit density of Ultrium 2 formatted tape is approximately 86% greater than that of the earlier Ultrium 1format. All things being equal, the change in areal bit density would nearly double the throughput specs of the new drives over the first generation of Ultrium drives. But all things are not anywhere near to being equal.

Obviously this is not an easy task and to solve it, the Ultrium 2 design employs a well-known digital signal processing technique in a PRML-4 (Partial Response Maximal Likelihood) read channel, which provides more accurate tracking than before. PRML technology originated at NASA to read weak signals from satellites deep in space. In essence, the channel compares the measured signal from the tape with a known waveform in order to correctly decode weaker signals and read/write data at a higher bit density.

As a result, the throughput specs for Ultrium 2 are double those of Ultrium 1. Throughput of streaming uncompressed data peaks at 30MB per second. In a typical backup scenario, expected throughput will be in the range of 50-to-60MB per second with compression. This means that each Ultrium 2-formatted half-inch cartridge will hold 200MB of uncompressed data and upwards of 400MB or more of compressed data.

What’s more, the Ultrium 2 format is designed to allow an Ultrium 2 drive to use older Ultrium 1 cartridges. The format of the tape is identified from the cartridge memory and that sets the drive's servo system to the appropriate format. The ability of read Ultrium 1 formatted tapes combined with a higher ATS speed range pegs top uncompressed data throughput on the Ultrium 460 drive using an older Ultrium 1 tape at 20 MB per second. This compares with 15MB per second for an Ultrium 230 drive.

To test the performance of the HP StorageWorks Ultrium 460 drive, openBench Labs calibrated the drive using its OBL tape benchmark. Our tape benchmark generates two very different types of data stream: purely random data, which is not compressible, and data that falls into a preset frequency pattern, which is explicitly designed to produce a compression ratio in the range of 1.8-to-1 to 2-to-1. We ran the tests on the same hardware running under both SuSE Linux 8.1 and Windows 2000 Server.

The OBL tape benchmark starts by allocating a large block of memory from which it then streams patterned or random data to the device. By streaming data directly from memory, the benchmark eliminates bus bandwidth contention with other devices. In this way, the benchmark more accurately represents the data transfer rate of the tape device than the overall system data throughput. The data can be streamed in block sizes of 2n KB, where n ranges from 0 to 8. This simulates the differences in the way backup applications read data off of a disk drive. In particular, high-end backup applications tend to use 64KB reads on Windows and use 128KB reads on Linux. For that reason, we chose to use 64KB data blocks on tests run on Windows 2000 Server and 128KB data blocks on Linux tests.

All testing of the HP StorageWorks Ultrium 460 was conducted on a dual-processor HP Netserver 1000. We used a QLogic Ultra160 SCSI Host Bus Adapter (HBA) to connect to the tape drive being tested.

The results proved to be quite interesting on a number of levels. First and foremost, while openBench Labs has long measured higher streaming I/O rates from RAID drives when running on Linux, when it came to measuring streaming I/O from tape, for all practical purposes, there was no difference between Linux and Windows.

This was definitely not the case with the Ultrium 460. Using fresh media, with data compression turned off and using 128KB data packets on SuSE, we measured uncompressed throughput for the Ultrium 460 to be 29.6MB per second. Using 64KB data packets on Windows 2000 Server, the throughput lagged by about 12% at 26.1MB per second. By way of comparison, using an SDLT 320 drive with the same equipment, we measured native throughput to be 15.6 MB per second on both Linux and Windows.

Turning the tape drive’s hardware compression circuitry on and sending packets of patterned data brought throughput up to 55MB per second on SuSE with a compression ratio of 1.86-to-1. On Windows 2000 Server we measured throughput at 48.9MB per second with a compression ratio of 1.87-to-1. These performance results easily placed the HP StorageWorks Ultrium 460 as the best-performing half-inch single-reel enterprise-grade cartridge drive.

Perhaps even more remarkable in their implication for real-world performance were the results measured when we tested our worst-case scenario: data compression turned on at the drive while it receives a stream of incompressible data. The most typical example of this comes when .gz or .zip files are included in a backup set. This event disrupts data flow and can cause a repositioning halt as the drive wastes its own internal cycles trying to compress the data. Typically, openBench Labs measures a drop in throughput of about 10% when streaming random data to the drive with compression on as compared to having no compression (i.e., native throughput).

On both of the Ultrium 1 drives tested by openBench Labs, an HP Ultrium 230 and a Seagate Viper 200, detection circuitry built into the drive signaled the drive to turn off compression and openBench Labs measured a drop of only .5% in throughput. On Ultrium 2 drives, all of the circuitry has been redesigned and separate ASICs are used for the interface, servo system, formatter, digital signal processor, and read/write pre-amplifiers. The result of all of this wizardry is no measurable loss in throughput during any of our tests sending incompressible data to the drive when compared to native uncompressed throughput.

For a final test, we ran the Ultrium 460 on Linux with an Ultrium 1 formatted tape cartridge. We measured uncompressed native throughput to be 19.9MB per second as compared to 14.2MB per second using the Seagate Viper 200. With hardware compression on and streaming a compressible data flow, we measured throughput rates of 37.9MB per second on the Ultrium 460 and 28.7MB per second on the Viper 200.

 Divorced from the constraints of disk drive performance and software overhead, the HP StorageWorks Ultrium 460 sets a significant high-water mark for backup performance. Clearly, backing up data from single spindle drives to the Ultrium 460 will be sheer folly when it comes to performance. Nonetheless, as we have seen in numerous RAID performance tests, insuring a minimum 60MB per second throughput rate off of a RAID 5 stripe set is not that big of a challenge.

The interesting challenge will come for software. For the first time in a very long while, an off-the shelf commodity tape drive has high enough throughput levels that will not mask inefficiencies or unnecessary overhead in backup software packages.