Messages in this thread | | | Date | Thu, 03 Apr 2008 20:25:00 -0600 | From | Robert Hancock <> | Subject | Re: Slow tape drive timeout |
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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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