Actually this is a generic issue:
One should be able to modify the default H/W configurations that are hardcoded into mutipathd's code. 

Specifically:
If I have a IBM 1814 RAID and I am not interested in using RDAC (that is hardcoded) - what should I do?

My expectations:
1) If I put my own device into the devices section of multipath.conf, with the 
   same vendor/product as one that exists in the hardcoded configuration - I  
   expect it to replace the default.
2) If I put have a default configuation I expect the properties included in 
   it to replace the properties of the hardcoded configuration. 

Thanks

Your expectations are half correct.
1. is correct.  If you add a device to the devices section with the same vendor/product as the default, it will override it. If it doesn't, this is a bug.
2. is probably incorrect, by "have a default configuration" do you mean "add a configuration option to the defaults section".  This does not override the devices section by design.

The may multipath works is the following.

1. Any configuration parameter you have in the multipaths section overrides all other sections.
2. Any configuration parameter in the devices section overrides the default section.
3. If no multipath or device specific configuration is specified for a parameter, it simply gets the default value (specified in the defaults section).

Multipath has a compiled in defaults and devices section.  Any configuration specified in multipath.conf in these sections overrides the compiled in values.

However, I have found a bug.  If you use the exact same vendor and product string that's in the hardcoded version multipath will do the correct thing. You
can check these strings be looking at

# cat /usr/share/doc/device-mapper-multipath-0.4.7/multipath.conf.defaults

or by running

multipathd -k"show config"

If you do not use the exact same vendor and product string, both device sections are present, and currently, multipath picks the compiled in one over the user
supplied one.  This is a bug.

Hi,

My problem can be described as followed:
My company is a supplier of a NAS device over off the chelf H/W.
Whe do not always know what storage array will be connected to the machines that run our software. We would like to define a default configuration using
tools available in any Linux, specifically linux device-mapper-multipath using tur or readsector0. RDAC is a third party multipath software, which needs to be installed over Linux.
I am running over an IBM 1814 an I have the following configuration in multipath.conf. 

defaults {
         path_grouping_policy    multibus
         failback                immediate
         user_friendly_names     yes
         path_checker            tur
         no_path_retry           3
         rr_min_io               10
         rr_weight               priorities
         polling_interval        10
         hardware_handler        "0"
}

I expect this configuration to override the configuration in the hardcoded device section - this does not happen:
multipathd takes the hardcoded values for IBM and uses RDAC instead.

Like I explained in comment #2, multipath doesn't work this way.  The defaults are for parameters that aren't configured by a more specific level of configuration.

Perhaps something like

devices {
  device {
    vendor ".*"
    product ".*"
    <your config here>
  }
}

would be more like what you want.  This should override all the device specific configurations, and use yours instead.  But, it doesn't, because of the bug I mentioned at the end of comment #2.  If I fixed multipath to choose the user supplied device configurations over the compiled in device configurations, would something like the above device section solve your problem?

Actually, there I must have made a mistake when I tested this earlier.  Looking at the code and retesting, multipath does indeed take the user defined device over the builtin device.