Friday Fine-Tune: HP 3000 DLT vs. DDS
August 4, 2017
Backing up enterprise-grade 3000s presents interesting choices. Back in the 1990s when the 3000 being built and sold by HP, DDS at first had only two generations, neither of which were reliable. A DDS tape used to be the common coin for OS updates and software upgrades. The media has advanced beyond a DDS-4 generation to DAT-360, but Digital Linear Tape (DLT) has a higher capacity and more reliability than DDS.
When a DDS tape backup runs slower than DLT, however, something is amiss. DLT is supposed to supply a native transfer rate of 15 MBps in the SureStore line of tape libraries. You can look at an HP PDF datasheet on the Ultrium SureStore devices certified by HP for MPE/iX at this link.
HP 3000 community partners such as Pivital Solutions offer these DLTs, At an estimated cost of about $1,300 or more per device, you'll expect them to beat the DDS-3 transfers of 5 MBps.
When Ray Shahan didn't see the speed he expected after moving to DLT and asked the 3000 newsgroup community what might be wrong. Advice ranged from TurboStore commands, to channels where the drives are installed, to the 3000's bandwidth and CPU power to deliver data to the DLT. Even the lifespan of the DLT tape can be a factor. HP's MPE/iX IO expert Jim Hawkins weighed in among the answers, while users and third-party support providers gave advice on how to get the speed which you pay extra for from DLT.
If you are using a DLT it likes to get data in a timely manner. Otherwise it will do the old 'shoe shine'. This means that other devices on the line can affect the bandwidth on the channel and starve the DLT. If you are using something like RoadRunner, then the CPU can be a real factor in this equation (especially single-CPU machines). So, you may not only want to check the statistics portion of the report, but monitor your machine during backup with Glance or SOS.
Gilles Schipper of GSA said that a TurboStore command is essential. "If you're using HP TurboStore, are you using MAXTAPEBUF option on STORE command?" MAXTAPEBUF and INTER can make a major difference, cutting a backup to DLT from 7 hours to under 2 by adding the parms.
HP's Hawkins said channel configurations of backup devices are key to ensuring that DLT tops the DDS speed:
Generally this shouldn’t happen. It might happen if the DLT and disc are on the same channel while the DAT/DDS was on a separate one. Might also happen with large numbers of small files on semi-busy system as some DAT are better at start/stop than DLT. If you are running STORE the STATISTICS option can give a broad indication of throughput for A/B comparison.
One simple piece of advice is to try a new DLT tape, too.