linux-kernel - Re: Slow tape drive timeout

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Date:	Thu, 03 Apr 2008 20:25:00 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Kai Makisara <Kai.Makisara@...umbus.fi>
Cc:	Carlo Nyto <carlonyto@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: Slow tape drive timeout

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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