Tape backup: Set DLT to beat DDS
April 10, 2008
Backing up enterprise-grade 3000s presents more interesting choices today than 5-10 years ago. Back then DDS had only two generations, neither of which were reliable for certain. A DDS tape used to be the common coin for OS updates and software upgrades. The media has advanced to a DDS-5 generation, but Digital Linear Tape (DLT) has a higher capacity and more reliability than DDS.
When a DDS tape backup runs slower than a 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 over at an HP PDF datasheet on the SureStore, even certified by HP for MPE/iX, at this link.
HP 3000 community partners such as Genisys and Bay Pointe and Pivital Solutions offer these DLTs, and Orbit Software has an "order with our backup software" option, too. But at an estimated cost of about $1,300 or more per DLT device, you'll expect them to beat the DDS-4 transfers of 5 MBps.
HP 3000 customer 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. 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 you pay extra for in DLT.
Dave Gale wrote in an answer that device configuration and CPU are potential problems:
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 support company GSA said that a TurboStore command is essential. "If HP TurboStore, are you using MAXTAPEBUF option on STORE command?"
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.
EchoTech's Craig Lalley, who's made a business out of upgrading HP 3000 storage devices, said that even when a DLT is moved to a different channel than the disk drive, you can do more. "The easiest thing to do is run the backup in the C-queue. Also, try turning software compression off."
Allegro Consultants' Stan Sieler offered a basic remedy. "I'd try a new DLT tape. I've found that helps at times."
3000 user Jack Connor testified to how much faster a DLT backup becomes with the best software parameters for backup commands. "MAXTAPEBUF and INTER can make a major difference," he said. "I recently had a backup to DLT cut from 7 hours to under 2 by just adding these parms."
We recently ran an article about ScreenJet's advice about large backups which skip the tapes altogether. STORE-to-disk (STD) counts on the reliability of a second disk mechanism, but DLT tapes have moving parts and magnetic properties, too. They just seem to cost a good deal more than disks which hold 40 times more than a DLT tape.