From: gab10@cd.amdahl.com (Gary A Browning) Subject: Re: Free software and the future of support for Diamond products Date: 23 Sep 1992 07:24:34 GMT
In article <1992Sep20.095850.11258@fcom.cc.utah.edu>, terry@cs.weber.edu
(A Wizard of Earth C) writes:
> A DOS machine is prety stupid -- it doesn't have to run other applications
> at the same time as a QIC-40/QIC-80 driver. It runs only the driver.
>
> The main deficiency of this under a UNIX or UNIX-like OS should be obvious:
> Rather than the driver being part of the archive software, and reading from
> a static file system image (ie: directly from the disk/file being backed up),
> a UNIX driver would have to give enough time to tar (or some other archive
> program) and the disk I/O subsystems (file system, VFS, system call
> interface,
> pager, and disk device driver) and still *always* be just about to
> write the
> next block when the tape drive needs it. The driver wouldn't have
> enough
> influenc on the scheduler to distinguish "tar" from "xtank", since
> both are
> user processes. Thus either an enforced single user mode with
> limited
> running of daemons, or suspension of all non-archive related processes
> during
> tape access would have to occur.
ISC 2.2.1 supports my Irwin Floppy Tape. Since this came in the form as a
new driver in its own patch, either the hooks to the scheduler were already
in the O/S or it is unnecessary (An interesting note is that the original
distribution claimed to support many different Floppy Tapes but did not.
I'll bet Irwin supplied the driver to ISC. The new driver makes not claims
at supporting any other manufacturers drives)
If I had to make a guess, I would say that the time critical operations are
performed in the driver without releasing the CPU. I believe the only thing
that can interrupt the 386BSD interrupt handler is another interrupt. If it
should raise the interrupt fence, you should not be interrupted. I would think
that ISC would be similar.
The Irwin Floppy Tape drive is not a streamer. It is necessary to preformat
the tapes into 512 bytes blocks. Each block can be written without impacting
the block after it on the tape. You can do random access operations. Think of
it as an incredibly slow floppy. The point is that if the data for the
next block does not arrive in time, the drive simply has to reposition
before
writing it.
--
Gary Browning | Exhilaration is that feeling you get just after a
| great idea hits you, and just before you realize
| what is wrong with it.