Multipath command output - Help with understanding output

[Bryn M. Reeves]

On Tue, 2009-10-13 at 18:23 +0100, Dan Track wrote:
> I've already got the following /dev/mapper/mpath0 and
> /dev/mpath/3600c0ff000d7ba4f4575b24a01000000. Can you tell me how I
> can reload the config and end up with /dev/mapper/san1?

That's a little bit strange; normally you'd expect the /dev/mpath
entries to follow the same naming as is used in /dev/mapper.

That said, the symlinks in /dev/mpath are nothing but trouble and it is
strongly advised that you don't use them for anything. The main problem
is that they can at times get out-of-sync with the device-mapper status.
This can lead to a range of problems such as failed booting (since the
correct device names don't exist at the point they should in the boot
process) to data corruption when a stale symlink ends up pointing to the
wrong multipath device.

This happens because the device nodes in /dev/mapper are managed by
libdevmapper and so are updated in-sync with the state of the devices in
the kernel but the symlinks are managed in userspace by udev and so
there can be delays between the device-mapper's state changing and the
corresponding symlinks getting updated.

> Also when running pvcreate should I run
> 
> pvcreate /dev/mapper/mpath0

Always prefer the names in /dev/mapper when working with multipath
devices.

Regards,
Bryn.