How to use block device as a primary storage

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Good morning, everyone
       I get a LUN from iSCSI, and then I want to use it as the primary storage 
for cloudstack. Also, I want hosts in cluster to use the LUN directly, rather 
than using NFS?? But all hosts in  cluster use  the same LUN in at the same 
time,  how do you deal with the problem of concurrent access ?
       I used RHCS and GFS2,but I found many problems. So I want to change a 
new way!

Thanks