Re: Framing Discussion

[Mohan Parthasarathy <Mohan.Parthasarathy@eng.sun.com>]

> > No. Why do you need a copy ? These buffers are consumed
> > by iscsi layer and may be returned later to the same
> > pool. These are just some details.
> > Why are they not interchangeable ?
> 
> I have a hunch that Mohan has never worked with local filesystem
> or block device code :-).  
>

But it looks like you have worked ..
 
> The reason is that SCSI code inside operating systems does not
> work that way.  Operating systems perform reads from SCSI devices
> by allocating memory at a layer above SCSI and then telling
> the SCSI driver to read data into the memory it has allocated.
> This is the case even when the operating system (e.g., filesystem)
> is the originator of the I/O.  The allocation operation cannot be
> moved into the SCSI driver without significant code changes to
> the OS (e.g., the device driver interface has to change, and then
> we can talk about the impact on the filesystem).  To keep
> existing code happy, the act of iSCSI "consuming" the data

So, you are designing a standard with possible TCP options
"to keep the existing code happy". It would be nice to state
this somewhere. 

> in the above scenario will include copying (or otherwise
> moving) it to the location at which it was supposed to arrive.
> 
> >  This is an
> > assumption not stated anywhere.  Is there
> > a list of other assumptions that is documented ?
> 
> I suggest looking at the SCSI/block device driver
> interface inside an operating system like Solaris,
> which I'm sure someone at Sun can explain :-) .

I can also read code if that matters :-)

> It's very different from the interface to network devices
> (e.g., no unsolicited inbound data).
>
You seem to have worked in networking :-)

-mohan

  
> --David
> 
> ---------------------------------------------------
> David L. Black, Senior Technologist
> EMC Corporation, 42 South St., Hopkinton, MA  01748
> +1 (508) 435-1000 x75140     FAX: +1 (508) 497-8500
> black_david@emc.com       Mobile: +1 (978) 394-7754
> ---------------------------------------------------
>