Re: iSCSI: frame formats

[Stephen Bailey <steph@cs.uchicago.edu>]

Sandeep,

> DataLen will now be max 8M/4M but then we dont wish to have large 
> iSCSI PDUs in any case.

This max size is getting the the point where I'm sure it'll be an
irritant.

I would like (but, in fact, a sure way to guarantee that it won't
happen is for me to like it :^) to view iSCSI PDUs as the expected
grain size at which you might have software involvement in an
otherwise hardware-driven iSCSI implementation.

For example, when a target is returning read data for multiple
outstanding reads, it might want to return a bit at a time from each,
and each `bit' should be an iSCSI PDU.  Clearly sending these bits
will be a software-level decision.  That certainly was the rational
for allowing multiple FCP DATA PDUs per read operation, and I naively
assumed similar logic was being applied here.

The alternative is to say that the hardware will do iSCSI PDU
chunking, but if that's the case, I expect that the header is, well, a
bit bulky.

I'm also incredibly unexcited about having data length be a multiple
of 4 bytes (if that's still in the cards).  There operations within
the SCSI command set which return arbitrary length data.  There are
perfectly nominal cases where you get less data than you requested
(e.g. inquiry & request sense).  Furthermore, the SCSI architecture
does not prohibit this, even though certain commands do, so it is not
for iSCSI to say anything about this one way or the other.

The problem with handling lengths that include padding comes when
you're trying to move the data into a buffer which is a non-multiple
length.  For example, if I ask for 22 bytes of inquiry data (with a 22
byte buffer), what can I say, at the time a PDU arrives that has 24
bytes?  It might have 21 or 22 bytes (or perhaps even, erroneously 23
or 24 bytes).  A data residual coming later will tell the software how
much was actually there, but it can't tell the hardware.  The typical
expectation of this type of transfer is that it will only overwrite
bytes of the buffer that are actually transferred, but having padded
lengths will not allow this.

The completely standard solution to carrying arbitrary data of
arbitrary length in an aligned transfer unit is to pad the transfer
unit but report the exact (shorter) length.  Another solution, used by
FC, is to carry a pad length.  In iSCSI, why bother---you've just
reintroduced added the 2 bits you were trying to remove?

The data length scenario is not comparable to IP header lengths, where
what is being carried is not arbitrary data.

Certainly, for iSCSI additional header segments (AHSs) you could
arguably use this cell length technique, since we can control what
we're carrying (AHSs that need exact byte lengths will have to be
internally self-describing) but frankly, I still think it's a bad
idea.

I can't understand why we're messing around with all these tricky
`solutions' to standard problems.  We should avoid the temptation to
get cute, and wholesomely provide the same capabilities as any other
SCSI transport.  Specifically:
  o allow long PDUs
  o carry exact data lengths

Steph