Thick: All storage space that you specify when you create a thick LUN is allocated up front, and the storage space is reserved on the volume. Snapshots, other LUNs, and shares on the volume cannot consume storage space that is reserved. The reserved storage space is displayed on the volume icon in orange. The size of the LUN is reported as the total storage space that you specify when you create the LUN. You cannot assign more storage space than the available non-reserved storage space on the volume. As data is written to the share, the used storage space is displayed on the volume icon in purple. Because thick LUNs reserve space, this can have implications when using snapshots and Replicate.
Thin: Even though you specify the size of a thin LUN when you create it, storage space is assigned on demand, instead of up front. This method greatly improves the utilization rate of the LUN, because storage space is assigned only as data is written to the LUN. However, the size of the LUN is reported as the total storage space that you specify when you create the LUN. As data is written to the LUN, the used storage space is displayed on the volume icon in purple.
A thin LUN lets you over-allocate its size. That is, you can assign a LUN size that is larger than the size of the volume. You can then expand the volume as needed (if necessary, adding disks in the process) without expanding the size of the LUN, and therefore without disconnecting users. Make sure that you watch the volume capacity of the volume on which the overallocated LUN resides so you do not run out of storage space unexpectedly.
Note: Because thin LUNs allow you to overallocate your available storage capacity, you need to carefully monitor your space usage. If you do not pay attention to this, then you may find yourself in a situation where the OS running the iSCSI initiator thinks that it has free space in the LUN, when no such free space exists in the ReadyDATA.