DDS4 drive on 28655A

[Sunil Kumar K.U.]

Hello All,

Do you people have a working set-up of a DDS4 drive connected to a 28655A card on K-Class servers? One of our sites is planning to connect an external DDS4 drive to a K-Class server via 28655A card. I want to know whether DDS4 drives are supported on this particular card.

Any thoughts?

Thanks/Regards
Sunil Kumar

[Pete Randall]

Sunil,

The 28655A is a single-ended SCSI card and I believe the DDS4 is going to require an Ultra2 SCSI connection.


Pete

[Pedro Cosmen]

Hello Sunil,

We have tryed this configuration and it doesn't work. Apparently all goes good but suddenly in the middle of the backup, multiple SCSI resets occur and the backup aborts. This happens because the DDS-4 is Ultra-SCSI and the card is Single Ended.

Regards

[Bill Hassell · Accepted solution]

The 28655A card is a SCSI+parallel print I/O card from the NIO or HP-PB family. It is a single-ended SCSI-2 interface card, while the DDS4 drive is compatible with low-voltage differential as well as single-ended SCSI-2 and SCSI-3. The DDS4 is a high-density, 68-pin wide SCSI connector while the 28655A has a high-density, 50-pin narrow SCSI connector, so you'll need an adapter cable.

The tape drive is capable of transferring an uncompressed data rate of 3 MB/sec (async) with bursts up to 40 MB/sec (sync), while the 28655A card can maintain 3 MB/sec in synchronous mode but only 5MB/sec in asynchronous mode. Since average compression can sometimes be 2:1, the tape may need at least 6MB/sec of data from the computer to keep the tape moving. That's beyond the limit of the 28655A card so data starvation may occur and tape must stop, backup, and retry the write operation (seconds of time), thus increasing tape/drive wear and reducing the throughput significantly. There is no way to predict how compressible all your data might be. If it is only slightly compressible (say, random raw data files) then you can expect a full-speed 3MB/sec backup, perhaps a bit more. With highly compressible data (say a database that is less than half capacity), the throughput may be less than 1MB/sec average with all the resync operations.

Since the K-class computers are so old, there is nothing faster except the high-voltage Fast Wide Differential, HV-FWD which is electrically incompatible with the DDS4. This is one of the difficulties in connecting new technology with old computers.

[Ted Buis]

That definitely isn't supported.

[Sunil Kumar K.U.]

Hello All,

Thanks for your responses.

Bill, Thanks for the detailed reply. The reason behind going for a DDS4 drive is that the total backup files size has grown more that the capacity of a DDS3 drive.(Also a DLT drive and its cartridges are expensive for this site)

The available backup window at this site is pretty long, so even if the bacup takes more time to complete, it is acceptable. Here the question is that wthether this is a feasible/supported configuration or not.

Thanks in advance for all the replies.

Sunil

[Bill Hassell]

While the backup window might be long enough, that DDS4 drive may need regular repair due to excessive wear. It's always tempting to match the tape drive to the backup size but in the last few years, increased capacity almost always requires a much higher throughput to keep the drive running as it was designed.

A better choice would be a DDS3 changer. fbackup can handle changing the tapes with a short script and now you can have full performance onto multiple tapes in unattended mode. eBay has DDS3 changers for $185-$200.

[Sunil Kumar K.U.]

Hi Bill,

Thanks for the reply. I got your concern on the low throughput and related problems if a DDS4 drive is connected to 28655A. Let me suggest the DDS3 autochanger to the customer.

Thanks everybody for your time.

Regards/Sunil Kumar