RE: Framing Discussion

[Black_David@emc.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 :-).  

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
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 :-) .
It's very different from the interface to network devices
(e.g., no unsolicited inbound data).
 
--David

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