Two DLT7000 drives in the same SCSI bus

[Andre Braganca]

I use Omniback 3.1 in a HP-UX server for backups in a A4853 DLT Library with 2 DLT7000 drives, in different SCSI buses. But I need to do a time-critical backup in a different server which only has 1 SCSI (UltraWide) card. Is there any problem in putting the 2 drives in the same SCSI bus ? Are there any issues besides the available bandwidth ? This way I could backup different filesystems in different tapes at the same time which would result in a faster backup.

Thanks in advance for your help,
AndrT

[Tommy Brown]

I can't speak for the technical aspects, but I used to run a DLT4000 and a DLT7000 on the same channel with no apparent problems. But, your comment regarding backing up different filesystems at the same time makes me question your backup datalist. Are these on the same server, if so then Omniback will bring up more than one diskagent to keep the data flowing to the DLT. If on multiple systems, put them in the same datalist, and the will keep the # of diskagents open until there are no more filesystems to backup.. I usually have 4 diskagents.
Tommy

[Rick Garland]

There is a document out there somewhere (I can't remember now) that explicitly states one DLT 7000 per one bus. This applies to the DLT 8000 as well. DLT 4000 are OK to chain.
The configuration you are dicusssing is not supported by HP. It is a question they will ask.

I was bit by this some time ago. The SCSI bus is flooded and I ended up with numerous backup failures, lbolts, etc...

1 DLT 7000 per SCSI bus.

[MR RAJIEV GUPTA · Accepted solution]

I just spent a full day establishing that 2 DLT7000 drives on single F/W SCSI bus do not work with T500 servers. Both tar, dd, and Omniback give errors.

Our Omniback jobs kept failing and this was finally the reason.

HP informs us that only PCI based SCSI cards can handle multiple DLT7000 drives, although the tapes might not stream.

We have bought a STK SCSI mux for our older servers (such as T500 and K-class), but now we have to limit it's use severaly.

[Ed Ulfers]

I have similar configuration (D370 running 10.20 with a 7448 Library and Omniback 3.1), 4 DLT7000 drives with 2 on each of 2 busses (HP unsupported configuration). Most of the time it works normally, but every so often when loading a tape on the same bus as an active drive the newly inserted tape fails to get to load-point within the expected time (causing a fail to load tape failure). I have not seen the problem if both tapes are initially loaded at the same time, before the data begins streaming to the drives.
Also, the bus throughput with two drives on one channel is MUCH lower than each drive on its' own channel.

Currently, I primarily use 2 drives on separate channels for backups and keep the second drive on each channel for tape copies.

Hope this helps.

[Ed Ulfers]

I have similar configuration (D370 running 10.20 with a 7448 Library and Omniback 3.1), 4 DLT7000 drives with 2 on each of 2 busses (HP unsupported configuration). Most of the time it works normally, but every so often when loading a tape on the same bus as an active drive the newly inserted tape fails to get to load-point within the expected time (causing a fail to load tape failure). I have not seen the problem if both tapes are initially loaded at the same time, before the data begins streaming to the drives.
Also, the bus throughput with two drives on one channel is MUCH lower than each drive on its' own channel.

Currently, I primarily use 2 drives on separate channels for backups and keep the second drive on each channel for tape copies.

Hope this helps.

[Joseph T. Wyckoff]

You might think of a SCSI buss as a collision domain sort of like Ethernet - there are some similarities.

If you try running both drives at the same time, you may see some intermittent failures.

Some are less horrible than others of course.

However, one kind of failure is a SCSI BUSS reset - which essentially will kill any backup operation involving that scsi buss.

A SCSI buss reset can result from a gross failure, or often from a series of minor errors. Often a hardware/proactive management utility will count the number of errors, and after reaching a threshold (per hour, per day, whatever) reset the SCSI channel.

My advice would be put in another scsi card.

Better yet - move/attach the tape drive to the server that has the data - thus reducing network load at backup time too.

Good Luck.

[Joseph T. Wyckoff]

Here is a trade-off for you...

You might find that using software compression will help with your two-drive, one card scenario.

That is, assume you have two globs of data, each 100mb in size.

With hardware compression, you send 200mb across the scsi channel, and the drive does the compression, quick, easy.

With software compression, Omniback uses CPU cycles to compress the data. Now instead of 200mb going across the scsi channel, you have maybe 150 MB going across the scsi channel. By the way - if you do this DO NOT use tape hardware compression - using both hardware + software can slow things down, use MORE tape, etc. Some things just don't compress twice.

Therefore, if the scsi channel is limiting the backup speed, and if you have more than a little extra cpu and memory, and if tape hardware compression is turned off, then you might be able to pass more data over the scsi channel, per unit time, by using software compression. Maybe.

Good Luck.

[Michael Lampi]

If your A4853A library is configured for Fast Wide Differential SCSI, then you will also need a Single-ended to Differential bus converter in order to have your library function properly on the new server.

Be aware that the DLT drives will only transfer data over the SCSI bus at the "fast" rate. This is at most 20 MB/sec.
Other peripherals on the bus will continue to transfer data at up to Ultra speeds.

Otherwise, the only other problem is if there are other "ultra" devices on the bus, then you might run into cable length problems. Keep the cables as absolutely short as possible, with the total cable length (internal and external) less than 1.5 meters. Be sure to use an active negation terminator, too.

[Andre Braganca]

Thank you all for your replies. Took what you've said, done some testing and also checked with HP and these are the conclusions that I found most helpfull:
- multiple DLT7000 in a single SCSI bus configuration is unsupported by HP;
- PCI SCSI cards (my case) can handle multiple drives but tapes might not stream, i.e. one can have the backup aborted;
- I've already done a backup with this configuration and everything was OK, so this can be a one-time solution but certainly not an everyday procedure;
- the use of software compression might help.

[Andre Braganca]

Just got another feedback from HP (for my configuration):

For maximum performance HP recommends a maximum of one DLT8000, DLT7000 or 9840 tape drive per SCSI bus. HP will support up to two of these drives per SCSI host bus adapter, but recommends only one per SCSI bus.