iSCSI:SCSI Mode Pages

Robert Snively rsnively at Brocade.COM
Wed Sep 19 10:30:48 PDT 2001


* From the T10 Reflector (t10 at t10.org), posted by:
* Robert Snively <rsnively at brocade.com>
*
Not quite.  As Gerry pointed out, whether the data is actually
buffered or is transferred to the media as it is provided
is invisible to SCSI/iSCSI and is managed by the target,
which is the expert on how it wants to go about storing
the data.  It is indeed likely that the target will ask for
the data in the order that it thinks will optimize the storage process,
which may require that the various groups of data that
are requested are transferred to the storage medium immediately.

There are no rules about how that buffering is performed.

There is no rule about how the buffer is allocated, and you cannot
make any assumptions in the protocol or the initiator about how
the target handles the buffer.

You cannot go out and look at the buffer (except for the
special case of buffered tape operations that have been halted
by certain special conditions, like a full tape).

All this stuff is handled as an abstraction by the SCSI
Architectural Model and by the transfer protocols, including
iSCSI, and the target can cheat any way it wants on implementing
that abstraction.  In fact skillful cheating is where most of
vendor specific value is added to the performance, cost, 
availability, and capacity of SCSI (and therefore FCP and iSCSI)
devices.

Bob

> -----Original Message-----
> From: Sanjeev Bhagat (TRIPACE/Zoetermeer)
> [mailto:iscsi_t10 at sanjeevbhagat.com]
> Sent: Wednesday, September 19, 2001 6:56 AM
> To: Robert Snively; Gerry.Houlder at seagate.com; t10 at t10.org; IPS
> Refelctor
> Subject: Re: iSCSI:SCSI Mode Pages
> 
> 
> 
>  I agree with Bob.So basically you would like to say that option no. 2
> referred by me was correct. The data is stored in external 
> buffers only and
> not trasnferred to the SCSI LU untill the buffer is full with 
> ordered data.
> 
> But now my question comes whether the iSCSI target will have a common
> externa buffer and it will allocate a part of  buffer for each SCSI LU
> inside or is the every SCSI LU going to maintain its own buffers.
> 
> Is it also possible for 2 different SCSI LUs to negotiate 
> EMDP bit 0 and 1
> 
> Sincerely,
> 
> Sanjeev Bhagat
*
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