Text File

Aucbvax.1397 fa.unix-wizards utzoo!duke!decvax!ucbvax!htwalton@LL-XN Sat May 23 03:30:24 1981 mag tape driver interface program Some years ago we wrote a rather fancy V6 mag tape driver because we did a lot of tape work. I am not sure that I would recommend the driver, because it was a bit large and of course did not use IOCTL's, and because it perhaps provides a bit more than really necessary. However, we have an interface program to the driver, which turns out to be just about the only program that needs to interface to any special IOCTL calls. The following is essentially the beginning part of this programs manual entry:

.so /usr/lib/tmac.tl .th SMT I 7/26/77 .sh NAME smt \*- set magnetic tape drive .sh SYNOPSIS % smt unit_number [ operation ... ]

operations:

backward sets drive so read command reads backward -backward sets drive so read command reads forward bpi=200 sets 7 channel drive density to 200 bits per inch bpi=556 sets 7 channel drive density to 556 bits per inch bpi=800 sets 7 channel drive density to 800 bits per inch dump sets 7 channel drive for 800 bpi dump mode eofcl sets drive to write end of file on close -eofcl sets drive not to write end of file on close errms sets drive to print error messages on system console -errms sets drive not to print error messages on sys console even sets drive to even parity file=m positions drive to beginning of file m = 1, 2, 3, ... fskip=0 positions drive to beginning of current file fskip=j positions drive to beginning of j'th file after this fskip=-j positions drive to beginning of j'th file before this help prints list of smt operations odd sets drive for odd parity ready if tape is not ready on drive, prints message and waits rec=n positions drive to before record n = 1, 2, ... on tape reset -backward dump eofcl errms odd retry -rewcl -short retry sets drive to retry failed read/write 10 times -retry sets drive not to retry failed read/write rew rewinds tape on drive rewcl sets drive to rewind on close -rewcl sets drive not to rewind on close shell tells smt program it is being called from shell short sets drive to ignore short (noise) records -short sets drive not to ignore short (noise) records skip=k skips tape forward k records skip=-k skips tape backward k records standard -backward dump eofcl errms odd retry rewcl -short stat print tape status stat=o interprets octal number from sys console error message unl rewind tape and take it off-line (unload) unlock unlock tape so others can use it vol move tape to end of volume mark (between to consecutive eof's) weof write end of file mark on tape weov write end of volume mark on tape wready if tape not ready for writing, prints message and waits

Smt was used in shell files to pre- and post-position and set options, and as a diagnostic tool for finding tape errors. There was a lock/unlock system for giving unique ownership of a unit to a particular person. If someone else had the drive the program asked if you wanted it, unless the shell option were present to indicate there was noone to ask. The stat operation was very useful: it told you in detail exactly what happened on the last tape operation on a unit: e.g. read obtained 35 bytes with CRC error from record 1764 file 2. The things that can go wrong with a tape are many, and stat is essential. The printing of error messages on the console was a complete loser. The driver kept a count of the number of tape marks previous to the current location, and the number of records since the last tape mark, provided one had not traveled backward over the last tape mark. A major feature of smt is that one simply does not have to be near the tape drive hardware in order to use it, except of course for the actual tape mount/dismount.

Although I do not recommend the driver per se, there are a number of reliability features it had that are worth noting.

First, there is a standard V6/7 bug: you cannot sleep at positive priority in an open routine, least the delete key lockup your device. This was corrected with a bsleep routine that was like sleep but returned to its caller with an error code instead of trapping back up the stack. This bug was ubiquitous in V6, and also unfortunately in V7: also, you cannot sleep at positive priority if you have called mapalloc in read or write, but sleeping a negative priority locks up a tape drive on which you tried to read a record from a blank tape, unless you build timeout into the driver.

Second, on writing a lot in cooked mode all the buffers got tied up. I guess V7's treating of writing to a rewinding tape as an error might fix this; in any case I'd almost recommend elimination of cooked mode. Our V6 driver went to the effort of allowing itself to use at most 2 buffers for writing. We had to modify getblk to do this.

Third, and this is important in many drivers, and unfortunately not in most UNIX drivers, a history is kept of the registers: those previous to loading a new command, those just after loading, and those after command termination. This history turns the driver into a diagnostic: almost essential since tapes and tape controls are rather flaky beasts. A trace mode can be set by a global variable, changeable using adb, to trace all commands with any error, or all commands, on the system console. This is for debugging flaky hardware.

Fourth, in the event of any unusual error, like UNIBUS overrun, a power clear was issued to the tape controller. This is also essential for any i/o controller: the old DEC tm-11 controller was left in an awful state by a UNIBUS overrun, in which it failed to execute the subsequent skip reverse without giving any error indication . In general controllers are not graceful about unusual failures.

The Berkeley IOCTL's look fine: I'd like to see the reliability features just indicated included in any driver, and a better register history available. Also, the driver might keep a better record of tape position, including perhaps the length in records of the last 10 files.

We have not yet moved the old V6 driver to V7. The complaints from users at ll-xn about the V7 driver have just about reached the limits of endurability, but at the moment we are trying to avoid UNIX kernel work. We can give sources for the old V6 stuff to anyone who is seriously interrested in working on them.

----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen <jgoerzen@complete.org> of http://communication.ucsd.edu/A-News/

This Usenet Oldnews Archive article may be copied and distributed freely, provided:

1. There is no money collected for the text(s) of the articles.

2. The following notice remains appended to each copy:

The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.