gotwols@warper.jhuapl.edu (Bruce Gotwols) (02/10/91)
I have recently acquired an enormous quantity of data on Exabyte tapes. I wrote several test programs for use on a VAX 3100 workstation running VMS to time the maximum possible rate to retrieve the data. One program was in FORTRAN, the other in C; using both conventional I/O (eg. fread in C) as well as several variants of QIO. In all cases I was very disappointed with the maximum data rate. I don't recall the exact number offhand, but it was less than 50 kbytes/s. Now an Exabyte is theoretically capable of sustaining an average rate approaching 246 kB/s, and in practice I have timed it at 200 kb/s routinely (more about this below). I traced the slowness using the conventional support to the inability of the DEC supplied driver to read more than one tape record at a time (1 record = 1024 bytes). Obviously with this level of overhead it's not surprising that the data rate is so slow. So my question to the readership is: can anyone suggest a way to read multiple records all in one call under VMS? (Please keep in mind that under QIO it may appear that this is possible, but all that seems to be returned is one records worth.) In the meantime I have concocted another less desirable solution. I now load DEC's generic SCSI driver and I have written some QIO rouitines that allow some rather primitive control of the Exabyte. These routines are 100% call compatabile with the custom written IBM PC routines that were used to obtain the data in the first place. This means the PC routines also run on the VAX-- a nice bonus. My average data rate is around 200 kbytes/s which is not bad.. If it's so great then why did I ask the question above? The answer is that this really is a primitive set of routines. My code doesn't depend on interrupts at all and therefore I have to poll the SCSI bus continually for certain opperations. What I have managed to do is turn the workstation into almost a single user single task at a time machine while the Exabyte is running this way. So my second question is: does anyone know of a well written driver for an Exabyte tape under VMS? Thanks, Bruce -- Bruce L. Gotwols Johns Hopkins University, Applied Physics Lab., Laurel MD 20723 Internet: gotwols@warper.jhuapl.edu (128.244.176.48) SPAN: APLSP::STR::GOTWOLS