steckel@Alliant.COM (Geoff Steckel) (10/17/88)
By my latest measurements, "device level" interfaces have a small (say 2 to 10%) speed advantage over an equivalent SCSI interface. Nowhere near the large amounts other posters have been mentioning. Rationale: everyone is concentrating on bus speed, not system speed. Real drives in real systems spend much of their time seeking or waiting for rotational delays. Since 3600 RPM is pretty much cast in stone for existing disks of any interface (yes, exceptions exist, but many of them are >>slower<< than 60 RPS), the average rotational latency is about 8 mS. About the fastest 'average' seek you can buy is 8 mS, with real (large, high priced) disks averaging about 12-16 mS. The SCSI overhead time (as measured on my small but cheap set of SCSI host adapters and bridge controllers) is less than 2 mS a command. Comparing disks of equivalent raw transfer rate (i.e. 5 to 5, 10 to 10 Mbits/sec) I consistently get less than 4% penalty for SCSI. Using a no-latency-read SMD controller and 10 MHZ drives, about 12%, pretty much what you'd expect if you cut out most of the 8mS latency. Note that even the no-latency-read drives have to verify at least one sector header before starting the read, since it doesn't know where it is yet. SCSI host adapters are being built faster and faster. Advertised (I haven't personally checked this) async transfer rates are up near 4 MByte/sec, right up past SMDE. Command interpretation delays are also shrinking as more useful functions are put into hardware. As IPI2 is coming into fashion (6 MByte/sec real 10 MByte advertised) the SCSI-2 is coming out of the committees (with 16 bit transfers) and about 6 Mbyte/sec. Choosing the correct interface is, as always, a SYSTEM decision, and should never be done in an effort to blindly optimize a single metric. geoff steckel(steckel@alliant.COM)
terryk@pinocchio.Encore.COM (Terence Kelleher) (10/19/88)
Good points on system overhead. Sorry I had not said it myself. It
is these basic physical delays tend to level all.
The > 1.5 MBPS async rates on SCSI are limited in actual use. The
speed slows significatly as cable lengths increase and the high speeds
are usually only available when the host and target use SCSI
controller chips from the same manufacturer. In real systems, async
will run between 1.2 and 1.5 MBPS, typical. Synchronous, on the other
hand, will run 4 MBPS, at any cable length and remain complient to the
specification.
The > 8 bit SCSI bus is probably a long way off. I don't think any
current SCSI controllers provide the capability of controlling the
extra data lines.
Terry Kelleher, Encore Computer
UUCP: {bu-cs,decvax,necntc,talcott}!encore!terryk
Internet: terryk%pinocchio@multimax.ARPA