Subject: [Ips] Overflow bit in scsi command response

From: "Smitha Narayanaswamy \(smithan\)" <[email-masked]>
Date: Wed, 23 Feb 2005 10:00:24 +0530

When would an iSCSI target have extra data to be transferred beyond
the initiator's Expected Data Transfer Length? What is the initiator
supposed to do if the overflow bit is set?

Thanks,
Smitha

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From: Julian Satran <[email-masked]>
Date: Thu, 24 Feb 2005 08:45:10 +0200

Smitha,

Data beyond expected Data transfer length should never be sent by the 
target and in case they are sent should be dropped by the initiator (the 
can't reach memory as the Expected Data Transfer Length is the user buffer 
length!).
The Overflow bit is meant to signal  that the total amount of data 
presented to the target (by whatever deice is there) in accordance with 
the CDB was larger than the Expected data length.
Here are some examples:

1- a disk from which you read and ask to read 1200 byte and direct the 
disk in the CDB to read 3 sectors and the sectors are 512 bytes each - you 
will get an overflow
2 - a tape from which you read a tape block (of a length not known to you) 
and ask to read 1200 bytes but the tape block is 1300 bytes - you get on 
overflow.

In any case you are not supposed to transfer from target more than 1200 
bytes and the initiator is not supposed to place data beyond the buffer!

BTW the underflow is the opposite case - the device has less data than the 
expected.

It is also worth remembering that the target is the master of the transfer 
and the initiator is not supposed to do any scoreboarding - the initiators 
ahs only to make sure that the transfer is within the buffer boundaries.

Julo

"Smitha Narayanaswamy \(smithan\)" <[email-masked]> 
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23/02/2005 06:30
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[Ips] Overflow bit in scsi command response

Thanks,
Smitha