In CentOS7, multipath no longer uses the getuid_callout to determine the wwid of
a multipath device. Instead is gets the value from the udev environment, using
the attribute defined by the uid_attribute configuration parameter.  Multipath
will issue warnings if /etc/multipath.conf defines getuid_callout in CentOS7, and
will use the built-in value for uid_attribute instead.

The way multipath determines if a user defined device configuration should
modify a builtin device configuration or create a new device configuration has
changed in CentOS7. By default in CentOS6, the user config only modified a builtin
one if the vendor, product, and revision strings of the user configuration
exactly matched the vendor, product and revision strings in the built-in
configuration. However, the vendor, product, and revision strings are
interpreted as regular expressions in order to determine which configuration to
use with a particular device.  In CentOS7 this same regular expression matching
is used to determine whether or not a user defined device config should modify
a builtin device config.  If the user config's vendor, product, and revision
strings regex-match a builtin device config's vendor, product and revision
regular expressions, then it modifies the existing config.  This means that in
CentOS6, user device configs sometimes needed to user regular expressions as
their vendor, product, and revision strings.  In CentOS7, these should be plain
strings.  On upgrade, "hw_str_match yes" will be added to the defaults section
of /etc/multipath.conf to force multipath to use the CentOS6 behavior, if
necessary.

The default multipath.conf file used on fresh CentOS7 installs enables
find_multipaths by default. This will not be set on upgrades from CentOS6.

A number of builtin configuration settings have changed in CentOS7.  These
changes were designed to improve performance in the general case.  However,
they may not be optimal for every configuration.  Users who were using the
builtin configurations for their devices should verify that the new defaults
still work well for them.