Promising extraordinary restore reliability

Ecrix's VXA-1 tape drive promises reliable restores and raises the price-for-performance bar that the company's midrange competitors need to beat. The list prices for this drive won't appeal to the home market, but the VXA-1's 33GB native (uncompressed) storage capacity and streaming transfer rate of 3MBps might appeal to companies that thought DLT and AIT were their only alternatives.

VXA technology uses an Advanced Metal Evaporated (AME) data cartridge similar to, but not interchangeable with, standard 8mm data cartridges. The way the heads write data to and read data from the tape distinguishes VXA tape drives from other technologies. Similar to Helical Scan technologies, the read and write heads spin a diagonal path across the tape, creating a diagonal track.

VXA technology writes data to tape in Discrete Packet Format (DPF) packets. To form each packet, VXA technology adds cyclical redundancy check (CRC), Error-Correcting Code (ECC), and addressing information to 64 bytes of user data and provides 387 packets per diagonal track. VXA technology can read the packets in any order to reassemble the data stream.

Nonsequential reading of packets is possible with OverScan Operation (OSO). Ecrix implemented OSO to ensure reliable read and restore of data. In OSO, four read heads scan the tape during the read process, and each head reads all data packets that pass by. When a tape becomes distorted from improper storage or when a different drive reads it, the tape might not align with the heads the same as when the original drive recorded the data. With OSO, at least one of the heads is likely to read the packet, which provides a more reliable restore.

Variable Speed Operation (VSO) is another technology Ecrix implemented in VXA-1. VSO encompasses two dimensions of speed: the speed of the tape as it passes through the drive and the delay between when the tape starts to move and when the heads can begin to read or write data. During write operations, the computer system frequently can't deliver data to the tape drive fast enough to write continuously. During read operations, buffers might fill faster than the restore application can process the data. In either case, the tape drive must stop and restart when the system can provide or accept more data. Streaming tape technologies need to rewind a section of tape, then restart forward tape motion, also called backhitching, to get the tape up to speed before the data transfer can resume. With VXA technology, no rewind is necessary. When idle, the tape drive can restart and resume writing or reading in 35 milliseconds (ms).

During a read operation, Ecrix combines VSO with OSO to improve restore reliability, even for imperfect media. VSO can slow the tape speed to give each read head more opportunities to read a damaged tape's packets.

During a write operation, one head writes the data and another reads the data back to verify a successful write. When the tape has a bad spot, which the inability to read the data immediately after a write signals, the VXA-1 immediately rewrites the data to the next spot on the tape. This functionality bypasses bad spots on the tape, which further improves restore reliability and reduces the need for a verification step in your backup job.

The drive has a rating for mean time between failures (MTBF) of 300,000 hours and a media life rating of 20,000 passes. VXA media is available in 170 meter (33GB native capacity) and 62 meter (12GB native capacity) lengths.

To test the VXA-1, I used a 450MHz Pentium II Compaq Deskpro EN with 64MB of RAM and two Western Digital WD Caviar EIDE hard disks. I used a wide-to-narrow (i.e., 68-pin to 50-pin) SCSI cable to connect the VXA-1 through an Adaptec 2944 Ultra Wide SCSI controller. The hardware installation process was no different from installing any other external SCSI device. I installed a tape driver from Ecrix. The VXA-1 masquerades as a popular tape drive (an Exabyte Mammoth), which is a common practice when tape drive technologies are new. To choose this type, you use the VXA Configuration Wizard. I called Ecrix's technical support to ask about the Configuration Wizard's functions. I reached a support person on the first call and soon had my answers.

Using VERITAS Backup Exec 7.3 with the options Write Checksums to Media and Hardware Compression enabled, I backed up and verified a 7GB directory structure containing 48,216 files that resided on an 8GB WD Caviar EIDE hard disk. Next, I restored an 807.3MB directory with 7932 files to a 4GB WD Caviar EIDE hard disk. The backup took about 46.2 minutes with an average throughput of 2.4MBps. The verification took about 45.6 minutes with an average throughput of 2.5MBps. Backup Exec doesn't compare data on tape to data on disk during a verification operation. The software scans the tape to ensure that the tape is readable and compares checksum information when you enable the option Write Checksums to Media for the backup. The restore took a little more than 7.3 minutes with an average throughput of 1.8MBps.

The processes didn't reach the rated speed, which isn't surprising because of the EIDE disks. Given the fast speed of the tape drive, the system is unable to feed the disks fast enough to keep tape operating at rated speeds. Watching the tape drive during the backup operation was interesting. The tape direction indicator never faltered, showing continuous forward tape motion. The write indicator flickered continuously but showed some hesitation when sounds from the disk signaled that more head motion was necessary to read the data. The write indicator and sounds from the disk support the fact that the system couldn't feed the tape drive fast enough and thereby limited throughput.

I reran the same set of tests, this time connecting the VXA-1 to a Gateway ALR 9200 server with four 400MHz Pentium II Xeon processors and using the same SCSI controller and cable. The ALR 9200 has a RAID 5 array consisting of four 10,000rpm hard disks and one other 10,000rpm hard disk not in the RAID array. In the first test, with Hardware Compression and Write Checksums to Media enabled, backup of the 7GB data set took 34 minutes with a throughput rate of 3.33MBps. Because of data compression, the overall backup rate was better than the VXA-1's 3MBps rated speed. Restoring the same 807.3MB directory to the RAID 5 array took 7.6 minutes with a throughput rate of 1.76MBps. The restore to the RAID 5 array was considerably slower than the backup speed and a bit slower than when I restored the same data to the 4GB EIDE disk. When I restored the data to the single SCSI drive, performance picked up a bit. This restore took 5.9 minutes with a throughput rate of 2.29MBps.

To test the VXA-1's native throughput, I ran the 7GB backup test with Hardware Compression disabled to write the full, uncompressed data stream to tape. As I expected, this backup was slow and took 46 minutes with a throughput rate of 2.46MBps (i.e., 82 percent of the tape drive's rated maximum speed).

I used a Quantum DLT 8000 tape drive, which has a 6MBps throughput rating, 40GB native capacity, and a $6000 price, to run the same tests. Quantum targets this tape drive at the enterprise market. In my test, the VXA-1 (rated at half the throughput) provided 70 percent of the DLT 8000's speed (i.e., VXA-1 took 30 percent longer to back up the data). The VXA-1 also provides 82.5 percent of the storage capacity at about 25 percent of the cost. So, if you need more capacity than the standard 8mm, DAT, or DDS tapes can accommodate, or the system (rather than the tape drive) limits your backup speed, the VXA-1 might be what you need.

Ecrix has a winner here. With the promise of highly reliable restore in the face of imperfect media, performance that can rival that of DLT, and an attractive price, the VXA-1 earns a must-try rating.

VXA-1
Contact: Ecrix * 303-402-9262
Web: http://www.vxatape.com
Price: $899 internal; $1049 external
System Specifications: 3MBps native transfer rate, 33GB native capacity, 300,000 hours mean time between failures, 20,000 passes on media
Corrections to this Article:
  • "VXA-1" (December 1999) incorrectly states that the reviewer used an Adaptec 2944 Ultra Wide SCSI controller to test the product. The reviewer actually used an Adaptec 2940 Ultra Wide SCSI controller. We apologize for any inconvenience this error might have caused.