HP3000/HP 3000 FAQ
Last Updated: Thursday, February 01, 2007 02:38 PM
DAT/DDS Drives and the HP3000
Short answer for the "cons": 0) HP usually calls theirs DDS, sneering at the lowly DAT. I think it's to distinguish the computer certified tapes from the audio tapes? 1) DATs have no significant error recovery mechanism. If you encounter a tape error on a DAT, the rest of that file is inaccessible. Subsequent files on the DAT might be accessible, if your software is smart enough to issue the appropriate "skip" commands PRIOR to encountering the bad tape area. 2) DATs can't be read past the current end-of-tape marker. If you have a full DAT (say, 1.2 GB), and accidentally insert it into a drive (write enabled) and write over the first 2 bytes... poof! The entire rest of the data is "gone". (For the *really* desperate, an unsupported desperation trick *might* suffice to get about 90% of the data back, with a lot of expensive work.) 3) Lack of feedback You can't look at a DDS while in use, and estimate how much tape has been used. You can't determine how many write-retries were successfully done. (True, SCSIDDS offers a peek at both values, but with difficulty, and not all DDS drives on HP3000s are SCSI.) 4) Slower per megabyte than reel-to-reel, I think. 5) Slow load/eject times (compare to thread-it-yourself reel-to-reel, and to some auto-load reel-to-reel). 6) Jamming in drive Pros: 1) capacity! 2) small size! 3) cheap!
You'll note that none of the pros/cons are HP3000 oriented, but are true on any system.
... we experienced consistently a fifty-percent wall-time reduction after switching from HP7980 reel-to-reel to the HP model 6400 4000 DC. That's the 120 meter variety with hardware compression. At first, we were continuing to have compression turned on in RoadRunner, but since we cannot turn off the hardware compression (5.0 push), it seemed redundant; we turned it off. The through-put improvement was a big surprise as the transfer rate is rated as significantly slower that 1/2" tape. No one can explain it, but it is not a fluke.
The DAT engineer said: "The bigger the better." Via HPFOPEN, MPE supports a 16K buffer, which is ok for DATs. STORE/RESTORE can go up to 32K. The basic criteria is to keep the tape streaming.
Yes! For DAT, the bigger the better. 32K will be good! Particularly for the compression drives, small blocks do not compress efficiently.
To prevent problems with your tape drive, keep the following list in mind:
* Are your tapes DDS certified? This is indicated by a DDS (Digital Data Storage) symbol on the tape.
* Do you replace your tapes on a regular basis? HP recommends tape replacement after approximately 300 uses.
* Do you clean your drives at regular intervals? Drives should be cleaned after 25 hours of use (Calculate this at approximately 2 hours for each tape use) or whenever the Cassette indictor light flashes the Caution signal (indicated by an alternating green light for 4.5 seconds and and no light for 0.5 seconds.)
* Do you replace the cleaning cassette at regular intervals? Each time a cleaning cassette is used, the date of use should be marked on the label. After 25 cleanings, the cleaning cassette must be replaced. If you fail to record each use of the cleaning cassette, after 25 uses the cleaning cassette may appear to be working normally, but it will be doing nothing. A working cleaning cassette will take about 54 seconds to clean the drive, a used up cleaning tape will take about 11 seconds.
* If you are having problems with a tape drive reporting that tapes are bad, and you suspect that the tapes are ok, HP recommends cleaning the drive three times in a row. If the problem still persists, place a call to have the drive replaced.
FWIW, the "needs cleaning" indication is completely different for the HP6400 model 4000[DC] drives, as are most of the other indicators.
Testing that we performed on backup technologies yielded the following results:
A HP7980XC delivers backup rates of approximately 600-650 KBS. A DAT drive delivers backup rates of approximately 300-350 KBS.
The testing was performed on a standalone HP3000 960 dedicated to the test. All stores were performed with TurboSTORE. The data was a copy of actual production data and spanned multiple tapes.
The HP7980XC was attached to a dedicated HP-IB card to ensure that the tape would operate in stream mode. Our testing indicates that 2 HP7980XCs can be connected to each HP-IB card and maintain stream mode. Data storage rates were approximately the same whether the drive was in hardware compression mode or not.
The DAT drive was attached to a dedicated SCSI card. Several DAT drives were tested. The transfer rates were approximately the same but amount of data stored varied.
Time for reel changes were not included in the transfer rate calculations even though it impacts the backup time.
We need to be a bit more specific about what rates and tape drives are wanted. RE HP DDS drives (note DDS is a subset of DAT) the 60 Meter and 90 M drives have a native transfer rate of 183KBytes/sec. The new 120 M drives have a 510KB/sec rate. The 7980's rate is 781KB/sec.
But for Backups using HP TurboStore (or other products) the capacity of the tape media, mounting and rewind times, channel capacity and usage, and compressibility of the data, and who does the compressing (hardware or software) must be considered.
For an actual data point, I have a 3000/947 which backs up about 17 million sectors (4.3 Gbytes) and it takes about 3.5 hours with software compression to a 90 M DDS. It uses about 2.4 hours to a 120 DDS with hardware compression. That is 1.25 GB/Hour and 1.83 GB/hr respectively. This may be somewhat limited by the tape and discs being on the same SE SCSI interface. Note that this is a low compression ratio as if I add only a few more files to the backup the 90 M DDS goes to two tapes.
For one reel STOREs the 7980XC is faster, but for the storage and convenience of one reel Full Backups, DDS with compression is King.
Only on N and A-Class systems
There is a url (public) you can go to get faqs on e3000 storage media;
The following site gives a list of Frequently Asked Questions about DDS3 and DDS4 tape drives and tapes:http://www.hp.com/products1/storage/products/storagemedia/tape_dds/faqs.html