SCSI problems and buffer underruns

Question: I keep getting "buffer underruns" or the SCSI hangs. What can I do about this?

Answer (by John Hendrikx <john@globalxs.nl>): I experimented with Buffer Underruns while reselection was turned off for my CD burner. This prevented the hard drive from doing anything while the CD burner used the SCSI bus (which can be for long periods at a time). Turning on reselection for both my burner and my hard drive occasionally led to SCSI errors. When I turned on reselection for my CD burner only, everything worked wonderfully. There was hardly any change in speed, since the hard drive (for which reselection had been turned off) did not use the bus for long anyway.

I have burnt CDs successfully at four-speed using the Yamaha writer, even though I have just an 030 system with 22 MHz and 8MB FastRAM. The data came from an IDE drive (I imagine that the additional IDE controller simplified things).

Here, nonetheless, are a few tips:

• If you feel that your hard drive is too slow, try running ReOrg.
• Use large block sizes on your image partition. I use 2 KB blocks on all image partitions (perhaps even larger values would help, but I like to use these partitions as normal partitions too. This reduces fragmentation effects on your hard drive.
• Try the reselection settings for your SCSI device. Reselection does not need to be turned on for all fast drives (e.g. hard drives) but it should be turned on for the CD burner (I have turned off reselection for all drives bar the burner -- works very well). If you want to see the difference that reselection makes, try the following :turn on parallel read/write, write a large image file to a CD (test mode). Now use a reselection tool and turn on/off reselection for the burner. The moment it's turned off, the buffer becomes smaller and smaller. Once it is turned on again, the buffer fills within seconds (things might possibly be different with your system, but try it anyway.
• If you suspect that your SCSI interface is simply too slow, or if turning on reselection for the burner leads to SCSI errors, try to read the data from a different SCSI interface (a second SCSI interface or an IDE controller).

Incidentally, turning off reselection for your hard drives even improves the speed (at least if only one of them is used at a time). I got up to 20% improvement.

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