I am pretty new to LVM, and need some advices. Would there be any advantage if I put directories like /var, /usr, /home on a different volumes if I had only one physical drive(my server is actually raid 5 with 5 drives, but they are logically one drive)? Or should I just have one gigantic volume and put all the directories under root?

free tune up http://www.pcpitstop.com

Separate slices for critical folders are the 'proper' way to set it up -- I think it's because if one slice goes belly up, it does not affect the others, unlike one large partition...but is redundancy already handled by your RAID array ??

I've also run into problems where the slices are too small & changing them later is a pain on a production system -- ie, make sure /var is big enough for mail, mysql DB's & app's that don't use /tmp for tmp files....

I normally just do one big partition.