Re: Slow tape drive timeout

[Kai Makisara]

On Thu, 3 Apr 2008, Robert Hancock wrote:

> Kai Makisara wrote:
> > On Wed, 2 Apr 2008, Robert Hancock wrote:
> > 
> > > Kai Makisara wrote:
> > > > On Tue, 1 Apr 2008, Carlo Nyto wrote:
> > > > 
> > > > > I am experiencing a two minute timeout open()ing a tape device when
> > > > > there is no tape in the drive.
> > > > > 
> > > > > open() with O_NONBLOCK succeeds immediately, however.
> > > > > 
> > > > This is how open() is supposed to work according to standards (e.g.,
> > > > SUS) if
> > > > O_NONBLOCK is supported. (Well, actually open() should wait indefinitely
> > > > but
> > > > the non-linux systems I tested had a timeout.) The linux st driver was
> > > > changed to comply with standards at 2.5.3. I.e., the 2.4 kernels did
> > > > return
> > > > immediately but the 2.6 kernels have always waited.
> > > > 
> > ...
> > > Why is accessing the tape drive with no tape in it causing a timeout in
> > > the
> > > first place? I should think that would fail immediately with some "medium
> > > not
> > > present" error from the drive. Unless the drive has no mechanism to detect
> > > it,
> > > but that seems really retarded..
> > > 
> > It does not seem retarded to me. If a program wants to just wait until the
> > tape drive becomes ready (e.g., loads the tape), it can use the blocking
> > open. This is simple. If a program wants to test the status, it uses
> > non-blocking open. The behavior mandated by the standards provides
> > alternatives. If O_NONBLOCK is not supported, the user program must
> > implement the waiting logic if the program just wants to wait until the
> > drive is ready before starting i/o.
> 
> This behavior is not consistent with any other removable storage device
> provided by Linux, however. If you try to open a CD-ROM device node when no
> disc is inserted, it doesn't block, it just gives you an error right away. Why
> should the tape drive behavior be different?

The tape driver supports O_NONBLOCK, the cdrom and disk drivers don't. The 
tape and disk drivers are otherwise so different (tape is character 
device, disks block devices) that I don't think it is a problem if the 
tape drive supports more features than the disk drivers.

I looked at my emails from 2001 when support for O_NONBLOCK was 
introduced. The reasons for this where standards (as mentioned already) 
and that some applications required this feature. So, it was not added 
just for fun.

-- 
Kai
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