text: When a thin-provisioned volume is created, a small amount of the real capacity is used for 
  initial metadata. This metadata holds a mapping of a logical address in the volume to a*grain*
  on a physically allocated extent. 
  
  When a write request comes from a host, the block address for which the write is requested is 
  checked against the mapping table. If a previous write to a block on the same grain exists as 
  the incoming request, physical storage was allocated for this logical block address and can be 
  used to service the request; otherwise, new physical grain is allocated to store the data, and 
  the mapping table is updated to record that allocation. 