Spindle is the axis on which the hard disks spin.
In storage engineering, the physical disk drive is often called a “spindle”, referencing the spinning parts which limit the device to a single I/O operation at a time and making it the focus of Input/Output scheduling decisions.
The only way to execute more than one disk operation at a time is to add more than one “spindle”; a larger number of independently seeking disks increases parallelism.
In computers spindling is the allocation of different files (e.g., the data files and index files of a database) on different hard disks. This practice usually reduces contention for read or write resources, thus increasing the system's performance.