I saw this, ( and more at Professionalvmware.com ) and I didn’t get a chance to post it right away, and there was some good discussion on it as well.
Many of you will say “RAID 5, of course!” and for those of you that do I would like to ask why? Is it because you really understand the differences between the different RAID levels? Granted that you may but have you ever had to do a RAID rebuild of a RAID 5 volume? How long did it take? Was it actually successful?
What RAID level would I use for my VMFS? RAID 10, of course! 😉
Needless to say this post is not about asking questions, but instead give you insight into my years of experience with this very subject. RAID 5 was an acceptable choice when the cost of hard drives were so high as to be prohibitive. However, these days quite the opposite is true. The benefits for using RAID 10 (performance, recovery, etc) far outweigh the costs for the additional drives required.
Art S. Kagel has written an excellent article about the different RAID levels (including RAID 3 and 4) as well as why he also chooses RAID 10 over RAID 5:
If you are currently using RAID 5 for you critical uptime production VMFS then you may want to consider creating a new RAID group, carve out a LUN and Storage VMotion your VM’s.
Check out my comment, as well as others. I also have a simular question about raid levels on my forums. Here.