What is this new /run filesystem?

Apparently, many tools (among them udev) will soon require a /run/ directory that is mounted early (as tmpfs). Arch developers introduced /run last month to prepare for this.

The udev runtime data moved from /dev/.udev/ to /run/udev/. The /run mountpoint is supposed to be a tmpfs mounted during early boot, available and writable to for all tools at any time during bootup, it replaces /var/run/, which should become a symlink some day. [1]

There is more detail here: http://www.h-online.com/open/news/item/Linux-distributions-to-include-run-directory-1219006.html

[1] From thread on the Arch Projects ML

The /run directory is the companion directory to /var/run. Like for example /bin is the companion of /usr/bin.

That means that daemons like systemd and udev, which are started very early in the boot process - and perhaps before /var/run is available (i.e. mounted) - have with /run a standardized file system location available where they can store runtime information.

Like /bin contains important programs, which may be needed in the boot process before /usr is available (in case it is on its own filesystem).

The /run idea is a relatively new idea/standard.