RE: Keep-alive traffic (was iSCSI: more on StatRN)

["Y P Cheng" <ycheng@advansys.com>]

StatRN and keep-alive are intended for detecting and recovering a lost
connection or iSCSI command.  My opinion is they are mandatory only if
Internet is a very unreliable connection. In traditional SCSI adapters, a
target device never initiate a recovery but must detect duplicated commands.
An initiator device always tries its best in detecting an error early and
reissuing the command without resort to big-hammer.  Application software
and device driver timeouts are imperative because a SCSI device can die
without warning.  StatRN and keep-alive are needed when the frequency of
losing a connection or command is so high that recovery by timeout is
considered undesirable and inefficient.  If the error frequency is so low,
then, keep the design simple and stupid by letting device driver timeout do
its job.  When a target device is shared by many initiators, kill it with a
big-hammer involves everyone sharing the target.  For those who debated
forever on hard and soft SCSI resets, they understand the need and
consequence of big-hammer.  For an iSCSI adapter, if the frequency of
connection and command loss is high, StatRN and keep-alive are useful in
helping detect the loss early.

To know why a target never initiates a recovery but must detect duplicated
commands and an initiator must detect an error early without resort to
big-hammer, please make a table of all possible errors and their recovery
actions, then, the conclusion is obvious.

Y.P. Cheng, CTO, ConnectCom Solutions Corp.