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Re: kern/43257: default SCSI-tape timeing parameters not selectable by kernel config

Hi, again.

If the parameters are settable during startup via sysctl.conf that would be
nice and may obsolete the ability to change the "global" kernal defaults.
But I think it does not hurt to make the kernel-defaults selecatable too as done at many other places too. (And that will enable the admin to work around tape-problems immediatly without other "still outstanding" kernel changes.)

Neverless it would be very smart to have these parameters available on a per device basis. These parameters are of cause tape-device dependent.
Accedently I have currently no idea how to integrate this.
It is no sollution - my oppinion - to make them selectable via ioctl or something like this, because no "normal" backup-system would ever use them or have a usable stub to set them. It does not realy hurt to have such a feature, but it introduces some semantic problems:
- when does a reset to the "default" values occure?
-- on every close? -> need to set them after every open again - not usefull
-- only with a special ioctl? -> possible unexpected side effects between different applications - may be problematic
-- after disconnect/reconnect of the device - e.g. after Timeouts?

The best way would be to use something like the sysctl interface for that - again my opinion.

If it would be possible to distinguish between "global" defaults for all devices and device-specific defaults in the sysctl interface that should be sollution for this problem. Sun has such a feature by selecting the device in a first call and accessing it in additional calls, but this is problematic and if two processes try to use it at the same time, wrong information gets retrieved an/or set ... I don't know much about the possibilities of the sysctl-interface in NetBSD in respect of accessing device specific parameters. The set of devices will change over time due to connect/disconnect of devices (eg. USB or after "scsictl scan"). I'm not shure if it would be a good idea to remember setting for device nodes accross disconnect/connect sequences. On the other hand, we will loose the setup for special devices after disconnect/connect if we drop them ... An other way may be to have different sets of default parameters depending on the device identification. But that requires more changes in the st-driver I think. And there must be an interface to control this during startup. (It makes no sence to have more than one set of default parameters compiled into the kernel. And to compile all setups statically into the kernel is not very user-friendly ...)

Best reguards,

W. Stukenbrock

Zitat von "Andrew Doran" <>:

On Thu, May 13, 2010 at 06:35:02AM +0000, David Holland wrote:
The following reply was made to PR kern/43257; it has been noted by GNATS.

From: David Holland <>
Subject: Re: kern/43257: default SCSI-tape timeing parameters not
        selectable by kernel config
Date: Thu, 13 May 2010 06:30:53 +0000

 On Wed, May 05, 2010 at 12:30:04PM +0000, Andrew Doran wrote:
> Thanks for the patch. On the approach: these should be tuneable via sysctl
  >  and not kernel options.

 Shouldn't they be device properties?

If the user can set them without recompiling the kernel, sure that
would work too.

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