gnu@sun.uucp (John Gilmore) (02/20/85)
> | Is it possible to find SCSI disk and tape controllers with reasonable > | performance? (Defined as delivering a useful fraction of the potential > | speed of the device, i.e. not requiring 5:1 interleaving on the disk > | because the stupid controller can't keep up.)... > > Surpisingly, 1:1 is possible. While earlier SASI/SCSI controllers were all of > the single-buffer type, newer (and still low-cost) boards from Adaptec, OMTI, > and (maybe) Western Digital are all capable of 1:1 operation (they look like > FIFOs). Note that most, if not all, SASI/SCSI disk controllers are too smart for YOUR own good. They "handle" bad block mapping but do a lousy job of it and won't let your software improve their job. For example, the Adaptec we use is utterly reliable and runs 1:1 interleaf, but: * Won't let you reformat part of the disk -- you have to do it all. * Won't let you remap a sector or track if it goes bad during normal operation -- you have to reformat the whole disk. (After dumping any data you want to keep, assuming your dumping tools make it past the bad block...) * Won't do automatic error checking & recovery. (Controller won't do the job itself either.) * Won't tell you where cylinder boundaries are so you can use fancy 4.2BSD code to optimize disk layout (only partly true -- it will tell you but you have to scan the disk to find out) All our SMD controllers give us full access to these facilities -- and we use them. We are looking for a controller (or Adaptec ucode upgrade) that will let us reformat & map sectors or tracks, or do slip sectoring.