originally NOPs where used to detect failures in the transport
layer. This is e.g. implemented in qemu since a few years now.
Recently I found a few vServers with hanging I/O where
the NOP mechanism could not detect the error.
The reason is most likely due to a bug in the target,
however if NOPs increase the CmdSN this could help
to detect such failures. If there are requests hanging
and the OS stops I/O before CmdSN > MaxCmdSN we can currently
send NOPs forever and they might still be answered. If
a NOP increases the CmdSN we will enter a point where CmdSN
is greater than MaxCmdSN. This is, of course, not the ideal check,
but it might detect some types of errors. Ideally we would
send out a Test-Unit-Ready command with attr=ordered, but
that requires a new API and/or modification of the tool
that uses libiscsi. This here comes with no modification for
the userspace.
Signed-off-by: Peter Lieven <pl@kamp.de>
5c4bd85b80