This is the number of bytes to be moved in the data transfer associated with the command. The direction of the transfer is indicated by 'dxfer_direction'. If 'dxfer_len' is zero then no data transfer takes place. 
If iovec_count is non-zero then 'dxfer_len' should be equal to the sum of iov_len lengths. If not, the minimum of the two is the transfer length. The type of dxfer_len is unsigned int.
Some HBA - SCSI device combinations have difficulties with an odd valued dxfer_len . In some cases the operation succeeds but a DID_ERROR host status is returned. So unless there is a good reason, applications that want maximum portability should avoid an odd valued dxfer_len .