cylinder head record format has been used by count key data (ckd) hard disks on ibm mainframes since at least the 1960s. this is largely comparable to the cylinder head sector format used by pcs, save that the sector size was not fixed but could vary from track to track based on the needs of each application. in contemporary use, the disk geometry presented to the mainframe is emulated by the storage firmware, and no longer has any relation to physical disk geometry.