It might be helpful to have the OS and version - one can only assume this is Solaris due to the md device (normally that's a Disksuite device). Posting the version of Disksuite would also be helpful.

mkfs is not normally used - as stated in the mkfs man page. It also matters if the device already exist and you are trying to make it larger (you can make it smaller but there is no "quick" way to do that).

Another consideration is if this is the root partition you are working with (or swap partition). There are set procedures to expand a root/swap partiton - they have to be followed to insure a viable system. You may want to check out info on sunsolve.sun.com (if you have an account) or
Disksuite info on Docs.sun.com