Re: Slow tape drive timeout

[Robert Hancock]

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?
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