grl@brb.dmt.csiro.au (Greg Lehmann) (02/14/91)
Does anybody have a DAT or 8mm tape drive that can do synchronous scsi transfers. Apparently most 1/2 inch tape drives support it these days. I am interested in the brand names of such beasts and in the performance compared to non-sync operation. What sort of throughput increase do you get when you start running synchronously? Also is there any improvement with the dreadfully long pauses that occur when you do things like mt -f /dev/rst1 rew or other operations that just cause slowdowns like tar cf /dev/rst1 ~mike ? Finally I still haven't heard a description of synchronous scsi and how it differs from ordinary scsi that satisfies me. Any pointers? TIA -- Greg Lehmann, CSIRO Division of Manufacturing Technology, Telephone: +61 7 365 3877 c/- Dept. of Mining & Metallurgical Eng., Facsimile: +61 7 365 3711 The University of Queensland, QLD 4072. Internet: grl@brb.dmt.csiro.au
courtney@tonto.es.com (Courtney Goeltzenleuchter) (02/15/91)
In article <grl.666498133@groucho>, grl@brb.dmt.csiro.au (Greg Lehmann) writes: > Does anybody have a DAT or 8mm tape drive that can do synchronous scsi > transfers. Apparently most 1/2 inch tape drives support it these days. > I am interested in the brand names of such beasts and in the performance > compared to non-sync operation. What sort of throughput increase do > you get when you start running synchronously? Also is there any > improvement with the dreadfully long pauses that occur when you do things > like mt -f /dev/rst1 rew or other operations that just cause slowdowns > like tar cf /dev/rst1 ~mike ? I don't know of any tape drives (DAT, 8mm or 1/2 inch) that support synchronous SCSI but I can say that it's unlikely it will make any difference. Data transfers are the only operations that are affected with synchronous. This could help the overall system throughput because the device is locking up the SCSI bus for shorter periods of time, but since most tape drives are quite slow (< 500kbs) speeding up the transfers isn't going to make much difference. For operations like rewind and forward space and the like, synchronous isn't going to make any difference. These operations are dependent upon the speed of the motor and/or the data transfer rate internal to the drive since very little data goes across the SCSI bus. > > Finally I still haven't heard a description of synchronous scsi and how it > differs from ordinary scsi that satisfies me. Any pointers? SCSI requires that every byte that goes across the bus be acknowledged by the receiver. In asynchronous mode that means the next data byte isn't sent until the ACK is received. In synchronous mode the sender is allowed to run ahead of the receiver ( a fixed number of bytes) allowing it to not wait for the ACK. The sender can continue transmitting bytes so long as the number of outstanding ACKs remains below a fixed size (usually 8-15). So the speed increase comes because we don't have to wait after each byte for the acknowledge before sending the next byte. -- Courtney Goeltzenleuchter, Evans & Sutherland Computer Corporation Design Systems Division, P.O. Box 8070, Salt Lake City, Utah 84108 (801) 582-5847 UUCP: { decwrl | utah-cs | sun!sunpeaks!sunslc } !esunix!cgoeltze INET: courtney.dsd.es.com
yuping@auspex.auspex.com (YuPing Cheng) (02/16/91)
In article <grl.666498133@groucho> grl@brb.dmt.csiro.au (Greg Lehmann) writes: >Does anybody have a DAT or 8mm tape drive that can do synchronous scsi >transfers. Apparently most 1/2 inch tape drives support it these days. The difference between synchronous and asynchronous transfer is that using the former you can get up to 5MB per second, while the latter allows you to get 1.5 to 3 MB per second depending on you cable length. But, don't forget your 8mm tape is writing data at 246KB per second, and your 4mm DAT is writing at 183KB per second. Hence the data transfer rate on the SCSI bus only changes the amount of time needed for fill the tape data buffer. If you have another disk device is sharing the SCSI bus with the tape device, the SCSI bus data transfer rate could be important, but not significantly. Because the tape device empties its buffer at its own recording data rate. Yu-Ping Cheng, Auspex Systems Inc., ycheng@auspex.com
dtb@adpplz.UUCP (Tom Beach) (02/16/91)
The summary says most of it! Wangtek's DAT does not appear to support syncronous SCSI, though. This 525 MB drive is a NICE drive! As to performance improvement with SCSI, you won't find that you can fill a tape any faster. What you will buy is more SCSI bus free time. If you can fill the tape drive buffer faster then it disconnects earlier and allows you some disk access while it empties it's buffer. The average data rate is, of course, determined by the tape speed and bit density. Tom B ------------------------------------------------------------------------ | Tom Beach : Sr Project Engineer : Mass Storage Technology | | phone : (503) 294-1541 | | email : uunet : dtb@adpplz.uucp | | ADP Dealer Services, ADP Plaza, 2525 S.W. 1st Ave, Portland OR, 97201 | ------------------------------------------------------------------------
olson@anchor.esd.sgi.com (Dave Olson) (02/16/91)
In <grl.666498133@groucho> grl@brb.dmt.csiro.au (Greg Lehmann) writes: | Does anybody have a DAT or 8mm tape drive that can do synchronous scsi | transfers. Apparently most 1/2 inch tape drives support it these days. | I am interested in the brand names of such easts and in the performance | compared to non-sync operation. Most of your questions were addressed by other responses, but none gave names. The ARDAT (Archvie) DAT drives support sync mode, except for the very early units. The primary advantage is not in sustained transfer rate (which is less than 183Kb/sec), but in overall SCSI bus utilization. This can be important if you have a number of SCSI devices on the bus, in that disks, etc. get more of the bus bandwidth. -- Dave Olson Life would be so much easier if we could just look at the source code.
kev@hpcpbla.HP.COM (Kevin Jones) (02/18/91)
> The average data rate is, of course, determined by the tape speed > and bit density. True for 1/2" technology. For 4mm and 8mm tape drives things get a bit fuzzy due to the ECC and format information embedded alongside "user" data. Also, for various reasons, these drives can "pad" data, further "diluting" the recording density (from the users viewpoint). In these cases, its no longer accurate to derive data rate by multiplying bit density by tape speed. From an overall "throughtput" standpoint: it is not uncommon that the "bottleneck" for the data rate to tape is the system itself. The speed of the disks, the power of the processor, the number of other things you're trying to do at the same time, and the amount of format information your "smart" backup application has to inject into the data stream can slug throughput such that the drive starts streamfailing. ----------------------------------------------------------------- Kevin Jones. | Hewlett Packard Ltd, | Computer Peripherals Bristol, kev%hpcpbla@hplb.hpl.hp.com | Filton Road, | Stoke Gifford, Tel: 011 44 272 799910 (ext 22351) | Bristol. BS12 6QZ. | ENGLAND. ----------------------------------------------------------------- This response does not represent the official position of, or statement by, the Hewlett-Packard Company. The above data is provided for informational purposes only. It is supplied without warranty of any kind.
ecr@cipric.mn.org (Edward C. Regal) (02/18/91)
In article <1991Feb14.181424.2010@dsd.es.com> courtney@tonto.es.com (Courtney Goeltzenleuchter) writes: >In article <grl.666498133@groucho>, grl@brb.dmt.csiro.au (Greg Lehmann) writes: >> Does anybody have a DAT or 8mm tape drive that can do synchronous scsi >> transfers. Apparently most 1/2 inch tape drives support it these days. >I don't know of any tape drives (DAT, 8mm or 1/2 inch) that support synchronous >SCSI but I can say that it's unlikely it will make any difference. Well, let's see - last I heard, the Exabyte and Wangtek DAT do synchronous. I would probably have to hook up the Ancot to be sure though. If you are looking for a really good 1/2" tape, try the HP88780A or S - we ran these here and were sorry we had to return the evaluation units. Ed -- *********************************************************************** * "Alll you people are soooo superstitious......." Firesign Theatre * * A strange thought remembered by Ed Regal ecr@cipric.mn.org * ***********************************************************************