Allows a cluster of computers to simultaneously use a block 
device that is shared between them (with FC, iSCSI, NBD, etc...). 
GFS reads and writes to the block device like a local filesystem, 
but also uses a lock module to allow the computers coordinate 
their I/O so filesystem consistency is maintained.  One of the 
nifty features of GFS is perfect consistency -- changes made to 
the filesystem on one machine show up immediately on all other 
machines in the cluster.