You can create a loopback or iSCSI file using dd. The following example creates a file with an initial size o 0 bytes and a allocated size of 8Gb.
dd if=/dev/zero of=LUN3 bs=1000 count=0 seek=$[1000*1000*8]
Mount the file using the following command.
mount -o loop /path/to/file /path/to/mountpoint