The /usr directory holds the majority of software content on a Red Hat Enterprise Linux system.
For an installation of the default set of software, allocate at least 4 GB of space. If you are a
software developer or plan to use your Red Hat Enterprise Linux system to learn software
development skills, you may want to at least double this allocation.

*** Do not place /usr on a separate file system:
If /usr is on a separate file system from /, the boot process becomes much more complex
because /usr contains boot-critical components. In some situations (like installations on iSCSI
drives), the boot process might not work at all.

Source: http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/pdf/Installation_Guide/Red_Hat_Enterprise_Linux-6-Installation_Guide-en-US.pdf