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Re: kern/43257: default SCSI-tape timeing parameters not selectable by kernel config
The following reply was made to PR kern/43257; it has been noted by GNATS.
To: "Andrew Doran" <ad%netbsd.org@localhost>
Cc: gnats-bugs%netbsd.org@localhost, kern-bug-people%netbsd.org@localhost,
Subject: Re: kern/43257: default SCSI-tape timeing parameters not
selectable by kernel config
Date: Thu, 13 May 2010 12:22:39 +0200
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 =20
too as done at many other places too. (And that will enable the admin =20
to work around tape-problems immediatly without other "still =20
outstanding" kernel changes.)
Neverless it would be very smart to have these parameters available on =20
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 =20
or something like this, because no "normal" backup-system would ever =20
use them or have a usable stub to set them.
It does not realy hurt to have such a feature, but it introduces some =20
- 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 =20
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 =20
that - again my opinion.
If it would be possible to distinguish between "global" defaults for =20
all devices and device-specific defaults in the sysctl interface that =20
should be sollution for this problem.
Sun has such a feature by selecting the device in a first call and =20
accessing it in additional calls, but this is problematic and if two =20
processes try to use it at the same time, wrong information gets =20
retrieved an/or set ...
I don't know much about the possibilities of the sysctl-interface in =20
NetBSD in respect of accessing device specific parameters. The set of =20
devices will change over time due to connect/disconnect of devices =20
(eg. USB or after "scsictl scan").
I'm not shure if it would be a good idea to remember setting for =20
device nodes accross disconnect/connect sequences. On the other hand, =20
we will loose the setup for special devices after disconnect/connect =20
if we drop them ...
An other way may be to have different sets of default parameters =20
depending on the device identification. But that requires more changes =20
in the st-driver I think. And there must be an interface to control =20
this during startup. (It makes no sence to have more than one set of =20
default parameters compiled into the kernel. And to compile all setups =20
statically into the kernel is not very user-friendly ...)
Zitat von "Andrew Doran" <ad%netbsd.org@localhost>:
> 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 <dholland-bugs%netbsd.org@localhost>
>> To: gnats-bugs%NetBSD.org@localhost
>> Subject: Re: kern/43257: default SCSI-tape timeing parameters not
>> =09selectable 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 =20
>> 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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