Re: iSCSI : digest error handling violates EMDP/InDataOrder

[Santosh Rao <santoshr@cup.hp.com>]

julian_satran@il.ibm.com wrote:
> 
> OK - I misread it. In any case we are not FCP and we don't violate iSCSI
> rules.

Julian,

What reasons exist to differ in EMDP behaviour b/n iSCSI and FCP ? 

Also, a fundamental question is that the description only speaks about
incoming data PDUs. Are you implying that InDataOrder only control the
ordering for READ data PDUs ? If so, what is the mechanism to control
ordering for write data PDUs ?

It is a useful control option for initiators to negotiate that R2T
requests be made in increasing continuous buffer offset order and R2T
requests not be sent out of order. Does iSCSI allow this ?

- Santosh

"31 InDataOrder 
    
   InDataOrder=<yes|no> 
    
   No is used by iSCSI to indicate that the incoming data PDUs can be in 
   any order (EMDP = 1). Yes is used to indicate that incoming data PDUs 
   have to be at continuously increasing addresses (EMDP = 0). 
    
   This also sets the Connect-Disconnect mode page EMDP bit. 
    
   The default is yes but targets MAY support no. "


------------------------------------------------------------------------------------

> > FCP uses it like iSCSI - i.e. the order has to maintained within a
> sequence
> 
> Not true. If you take a look at FCP-2 rev 04 Section 10.1.1.7
> description on EMDP, it explicitly states :
> "The EMDP bit does not affect the order of frames within a sequence".
> 
> For a WRITE command, an EMDP setting of 0 implies that the buffer offset
> in R2T requests must be in continuous and increasing order whereas an
> EMDP setting of 1 implies the buffer offset in R2T can be out of order.
> 
> For a READ command, an EMDP setting of 0 implies the buffer offset in
> READ data PDUs is in continuous and increasing order, whereas, an EMDP
> setting of 1 implies buffer offset in READ Data PDUs can be out of
> order.
> 
> Based on the above rules, iSCSI is violating EMDP setting by its error
> recovery for data digest errors detected by targets on Data PDUs.
> 
> - Santosh
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