The optimization type is either space or time.
Space - When you select space optimization, disk blocks are allocated to minimize fragmentation and disk use is optimized.
Time - When you select time optimization, disk blocks are allocated as quickly as possible, with less emphasis on their placement. When there is enough free space, it is relatively easy to allocate disk blocks effectively, without resulting in too much fragmentation. The default is time.
You can change the value of the optimization type parameter for an existing file system using the tunefs command.