Messages in this thread | | | Date | Fri, 4 Apr 2008 21:04:06 +0300 (EEST) | From | Kai Makisara <> | Subject | Re: Slow tape drive timeout |
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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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