RE: iscsi : DataPDULength can differ in each direction.

[Dave Sheehy <dbs@acropora.rose.agilent.com>]

Comments in text.

>   >  > 	Can someone give a tangible benefit to this that can
>   >  outweigh the
>   >  > spec and implementation churn at this late stage of the game?
>   >
>   >  It would allow iSCSI HBAs to interact more efficiently
>   >  with SW iSCSI
>   >  implementations and vice versa.
>   >
> 
> I don't believe it would in practice. Consider the following. The max
> PDU size sent during login is more that just that, it is in fact the
> senders maximum supportable max PDU size. If one side sends 64k and
> the other side 8k although it is technically indicating it can't
> receive more than 8k in a single PDU, for all practical purposes it is
> also indicating it can't handle, and therefore can't send PDUs bigger
> than 8k.

I think that's a faulty interpretation of what's being proposed. As in both
Fibre Channel and TCP the offered PDU size is the size you're willing to
accept. It can and often is completely decoupled from the size you're capable
of sending. This is commonly implemented in Fibre Channel and it's not 
exactly rocket science.

> I believe if we go this route we'll simply see the side with the lower
> DataPDULength sending its "natural" size PDUs and never sending the
> larger size wanted by the other side. More on this below ...
> 
>   >  > 	From my point of view the benefit of asymmetric PDU
>   >  sizes would have
>   >  > to be very large to make it worth the extra complexity
>   >  in buffer
>   >  > management code alone.
>   >
>   >  >From the vantage point of an iSCSI HBA it doesn't seem
>   >  all that hard.
>   >
> 
> Well, it seems to me faced with a peer with a different max PDU size
> there are relatively few ways to proceed.

Again, I believe it's fairly straightforward and there are real life 
examples that work exactly this way. The only unfortunate thing is that it's
been proposed at this late date. In hind sight, it's a feature with an
obvious benefit.

> If the peer has a lower max PDU size there are 2 choices. Use 2 buffer
> pools, one for receive set to the local Max PDU size, and one for send
> set to the peer Max PDU size. This is where the extra buffer
> management complexity comes in. Or, use one buffer pool and simply
> part fill the buffers for sending. This is the easy case.

I don't see what buffer size has to do with PDU size that's not implementation
dependent. The PDU content is going to be fragmented anyway (i.e. network
header, iSCSI header, data, digest, ...) so the problem you describe exists
regardless.

> If the peer has a larger max PDU size then either only send up to the
> local PDU size, as I mentioned above, or chain buffers together to
> build larger than the local max PDU size. Again, this is where the
> extra buffer management complexity comes in. Remember that by
> definition these chains will need to be bigger than the largest chain
> size the implementation can handle. Unless for some reason the
> DataPDULength sent was chosen at some arbitrary size smaller than the
> implementations maximum.

I think SW iSCSI stacks are going to want to use big PDUs and in general
iSCSI HBAs are going to use small ones. I think the SW stacks are going to
set up their buffer structures to use large PDUs. I don't think (although
it's certainly possible) that they are going to dynamically adjust their
buffer sizes for each iSCSI session. So, in the scenario where a SW iSCSI
stack is talking to an iSCSI HBA it's buffering is going to be used
inefficiently in both directions. If the Data PDU Length is negotiated
separately for each direction then the buffering on the SW side is being
used inefficiently in only one direction. Both sides win with this behavior.

Dave