I'm not familiar with MogileFS, so if my comments below are incorrect when applied to MogileFS, then I'm sorry.
GFS/HDFS are officially called filesystems, but they're about as much as a filesystem as a Samba server: they expose files to the outside world, with some extra layers of abstraction you can mount the filesystems under Linux, but in essence they live on top of existing filesystems themselves and have nothing in common with the traditional filesystems, where you operate on a block device. A SAN, otoh, is such a block device and traditional filesystems operate on them.
This means that you can run a RDBMS on a SAN, while you can't on GFS/HDFS. This covers a common use case where you have a central expensive database server, with dozens of terrabytes of storage in a SAN, which you see in a lot of large corporations.
GFS/HDFS are officially called filesystems, but they're about as much as a filesystem as a Samba server: they expose files to the outside world, with some extra layers of abstraction you can mount the filesystems under Linux, but in essence they live on top of existing filesystems themselves and have nothing in common with the traditional filesystems, where you operate on a block device. A SAN, otoh, is such a block device and traditional filesystems operate on them.
This means that you can run a RDBMS on a SAN, while you can't on GFS/HDFS. This covers a common use case where you have a central expensive database server, with dozens of terrabytes of storage in a SAN, which you see in a lot of large corporations.