iSCSI data transport packet overhead

Question

This question is admittedly overlapping with iSCSI Packet Header but I hope for a more specific answer than provided for that one for my more specific question.

Question: How much overhead is introduced by iSCSI and SCSI in the transfer of the data to read or write?

I understand that this is done in the Data-In and Data-Out transport packets and I understand there is overhead from TCP/IP and Ethernet, but what additional overhead is needed by iSCSI/SCSI? I recognize that SCSI Protocol Data Units (PDU) are packed into TCP datagrams by some algorithm, but I'm unsure of that algorithm.

I realize there is a full conversation that needs to happen between the initiator and target and I will ignore that part. So, I only want to know about the data packets sent following a read or write command.

Answer 1

For a read the below is correct. But for a write (if immediate data is not enabled) there are extra bytes required for the target to send another PDU called R2T which is 48 bytes. The digest is rarely used because most iSCSI connections are local and TCP/IP has a CRC.

Answer 2

Here is what I have assembled from looking through the iSCSI RFC 3720.

The Protocol Data Unit (PDU - iSCSI "packet") has these elements:

1. Basic Header Segment - 48 bytes
2. Additional Header Segments - optional - 0 bytes (assume these are not needed)
3. Header Digest - 4 bytes (optional, but want this)
4. Data Segment - 8192 bytes (the max received data length is negotiated between the initiator and target, 8192 is the default, so will use that)
5. Data Digest - 4 bytes (optional, but want this)

So, I see an overhead of 56 bytes to transmit 8192 bytes of actual data.