linux-kernel - Re: Slow tape drive timeout

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Message-ID: <alpine.LSU.1.10.0804032121380.9918@kai.makisara.local>
Date:	Thu, 3 Apr 2008 21:34:09 +0300 (EEST)
From:	Kai Makisara <Kai.Makisara@...umbus.fi>
To:	Robert Hancock <hancockr@...w.ca>
cc:	Carlo Nyto <carlonyto@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: Slow tape drive timeout

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.

-- 
Kai
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