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Re: [dm-devel] bugs in handling of errors for SG_IO and SCSI_IOCT L_SEND_COMMAND ioctls to block device



On Fri, 08 Jul 2005 11:48:08 -0500
Mike Christie <michaelc cs wisc edu> wrote

> Subject: Re: [dm-devel] bugs in handling of errors for SG_IO and
> 	SCSI_IOCT	L_SEND_COMMAND ioctls to block device
> To: device-mapper development <dm-devel redhat com>
> Message-ID: <42CEAE48 9050203 cs wisc edu>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> goggin, edward wrote:
> > On Thu, 07 Jul 2005 23:19:57 -0500
> > Mike Christie <michaelc cs wisc edu> wrote
> > 
> > 
> >>>...
> >>>
> >>>	The bio handling for these REQ_BLOCK_PC requests shouldn't be
> >>>treated any
> >>>	differently than the more typical REQ_CMD type block io request.
> >>
> > 
> >>what is meant by this last comment specifically?
> > 
> > 
> > Just trying to make a case that a device pass through read command
> > issued to a block device should transfer to user space only the
> > user requested number of bytes minus the residual from the transfer
> > (as is done for block device read and write requests) and not always
> > transfer the number of bytes requested by the user.
> > 
> 
> ah ok, I thought it was referring to something else, nevermind.
> 
> Was the reason it does SG_IO reads becuase opens on the 
> device were not 
> allowed?

Sorry, I should not have singled out pass through read commands.  My
statement above should have simply referred to "pass through commands
which are transferring data from the device".

> Was not being able to open a dm device a bug or by 
> design? 

I didn't provide my context for why the pass through commands are issued.
These are test path ios initiated by the multipathd in user space --
either tur, read of sector 0, or a page 0xc0 inquiry.

> For devices that do not support SG_IO like NBD, dasd and if you do 
> dm-multipath with AOE, another solution may have to be found.
>

Good point.  While dm-multipath is designed to be block device
protocol agnostic, its current implementation only supports
SCSI devices.


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