rpinder@phad.hsc.usc.edu (Rich Pinder) (12/01/90)
Some of the higher capacity SCSI disk drives rate transfer speed in both
Synchronous as well as Asynchronous, with the former being rated as much
faster. Is synchronous transfer possible on an Intel based box?? I'm
wondering if a NCR 486 microchannel, with the NCR 53C700 SCSI chipped
controller, running SCO Unix could address these 5.0 MB/second speeds they
claim? Is it true that a limitation to Synchronous transfer is that there
can only be one device running synchronous in a system??
thanks for any help.
Rich Pinder
USC School of Medicine
(213) 224-7099
rpinder@phad.hsc.usc.edu
||||||||||||||||||||||||||||||||||||||||||||||||||esmith@goofy.apple.com (Eric Smith) (12/03/90)
In article <28543@usc> rpinder@phad.hsc.usc.edu (Rich Pinder) writes: > Is it true that a limitation to Synchronous transfer is that there > can only be one device running synchronous in a system?? No. In fact, you need at least 2 devices that support synchronous SCSI operation before you see any advantage at all. -- Eric L. Smith Opinions expressed herein do not necessarily reflect those esmith@apple.com of my employer, friends, family, computer, or even me! :-)
taylor@chris.Solbourne.COM (Dick Taylor) (12/06/90)
In article <28543@usc> rpinder@phad.hsc.usc.edu (Rich Pinder) writes: >Some of the higher capacity SCSI disk drives rate transfer speed in both >Synchronous as well as Asynchronous, with the former being rated as much >faster. Is synchronous transfer possible on an Intel based box?? I'm >wondering if a NCR 486 microchannel, with the NCR 53C700 SCSI chipped >controller, running SCO Unix could address these 5.0 MB/second speeds they >claim? Is it true that a limitation to Synchronous transfer is that there >can only be one device running synchronous in a system?? The transfer rates they quote are, of course, referring only to their speed on the SCSI bus, and not to the throughput available from the drive. The limit to throughput is still the media data rate, and there aren't any drives out there with prolonged throughput anywhere near 5.0 MB/s. (Or rather, there aren't any single-spindle readily-available reasonably-inexpensive drives at that rate; there are a number of leading-edge products with sustained transfer rates in excess of 5.0 MB/s, which is one of the reasons for the push to faster and wider SCSI buses with SCSI-2.) The advantage of synchronous transfer is that the actual bus occupancy by any one device is much lower (the data is buffered, and intelligent devices will disconnect and clear the bus while they don't need to use it). This allows you to put more devices on the bus without contention problems. The disadvantage of synchronous transfer is that both the host and the device have to support it. Synchronous support is routine for modern high-capacity disk drives. It's not always routine for hosts, but a 53C700-based design certainly has the hardware to support synchronous transfers. As for the software, I happily refer you to your vendor's support department. > > Rich Pinder > USC School of Medicine > (213) 224-7099 > > rpinder@phad.hsc.usc.edu Dick Taylor Solbourne Computer, Inc.
billbr@xstor.UUCP (Bill Brothers) (12/18/90)
In article <1990Dec5.182853.25111@Solbourne.COM> taylor@chris.Solbourne.COM (Dick Taylor) writes: >In article <28543@usc> rpinder@phad.hsc.usc.edu (Rich Pinder) writes: >>Some of the higher capacity SCSI disk drives rate transfer speed in both >>Synchronous as well as Asynchronous, with the former being rated as much >>faster. Is synchronous transfer possible on an Intel based box?? I'm >>wondering if a NCR 486 microchannel, with the NCR 53C700 SCSI chipped >>controller, running SCO Unix could address these 5.0 MB/second speeds they >>claim? Is it true that a limitation to Synchronous transfer is that there >>can only be one device running synchronous in a system?? > >The transfer rates they quote are, of course, referring only to their >speed on the SCSI bus, and not to the throughput available from the drive. >The limit to throughput is still the media data rate, and there aren't >any drives out there with prolonged throughput anywhere near 5.0 MB/s. The main problem is that no matter how fast your host adapter and drive are, you still have to get the data jammed onto the bus and into the memory. We have reached speeds of 4.7 Mb/sec here in the lab under VERY controlled situations using bus-mastering style of host adapters and fairly high speed disks. We have found that synchronous support really doesn't buy you very much. We support it, but the average benchmark won't show much (if any) differences. If however, the limiting factor was the host-adapter, or the drive itself, synchronous equipment would be a factor. For example, using a proprietary bus-mastering style of host adapter on an EISA bus with a 486 does show some differences. The main problem that you are seeing has to do with the fact that the UNIX io-subsystem is CPU bound... Yes, you read correctly. There is sooo much overhead in the UNIX filesystem, 500-800K/sec is the max you can really expect on a 33 Mhz 386 system running SCO UNIX 3.3 Release 2. (Release 1 is about half that). There are some expensive ways to get more, but so far those are too expensive for the average joe. SCO has done a good job of working on the disk performance. Analysis show that clustering is in effect, and that the average disk request is now 8-16K instead of the old 1K UNIX subsystem. Depending on the drive selected, maximum throughput is approached somewhere in the 64-128K request size. Even on a buffer flush, the kernel takes 300-500 microseconds (on a Compaq systempro) to push the next request out to the driver. On UNIX the best performing subsystems are those that can provide the highest transaction rate. In other words, access time and SCSI overhead are your worst enemies. Bill Brothers Storage Dimensions, Inc. uunet!xstor!billbr