RE: Avoiding deadlock in iSCSI

[Pierre Labat <pierre_labat@hp.com>]

From the discussion going on the symmetric versus asymmetric model and
dead lock avoidance, here is  a proposal that tries to combine (as far
as possible...)
the advantages of both models.

The proposal is a slightly modified symmetric model where
the symmetric model incorporate an idea of the current
asymmetric model.

This idea is that the initiator indicates to the target
on which connection the next command in order arrives.
The asymmetric model does that using an extra connection
transporting the commands, and for each command in the header
there is the connection number where the data will come.


Description of the proposal:
---------------------
In the command header a field is added. This field
"next connection" indicates the TCP connection (connection id)
in the session on which the next command will arrive.
This field builds a linked list of commands across the several
symmetric connections of the session.

The first time the initiator posts a command, it determines
the connection on which it post the command and the connection
on which it will post the next command.
Then for each command the initiator determines only
the connection it will use to post the next command.
For each command, the connexion id of the next command is added in the
header
field "next connection" of each command.


Doing that, leads to the same algorithm as in the asymmetric model
to process the commands on the target side:
- a thread of execution pulls the cmd/data in order from the TCP
  windows. After pulling a cmd/data, it looks at the "next connection"
  field and move to the next connection to pull a new cmd/data.
  There is no need of extra storage out of the TCP windows
  and no need to re-order. In the asymmetric model the thread
  empties the command connection, here it follows the chain of commands.

The advantage of this proposal are:
- on the target, as opposed to the original symmetric model, it avoids
  the re-ordering process of the commands once extracted
  off the TCP window.
- it keeps cmd/data/status on a same connexion avoiding
  the synchronization penalty for the READS on the initiator (described
  by Somesh Gupta)
- it saves one TCP connection compared to the asymmetric model.


Anyway any ordering implementation adds overhead, thus, as in most
cases (disks) it is not required, It would be possible to deactivate it.



Regards,

Pierre