the information flow at constant rate along the spiralthe drive must speed up the rate of disk spin as the / head moves toward the center of the disk this makes for more complicated and slower mechanism three separate steps take place when reading particular byte or series of bytes of data from hard disk firstthe / head moves so that it is positioned over the track containing the data this movement is called seek secondthe sector containing the data rotates to come under the head when in use the disk is always spinning at the time of this writingtypical disk spin rates are rotations per minute (rpmthe time spent waiting for the desired sector to come under the / head is called rotational delay or rotational latency the third step is the actual transfer ( reading or writingof data it takes relatively little time to read information once the first byte is positioned under the / headsimply the amount of time required for it all to move under the head in factdisk drives are designed not to read one byte of databut rather to read an entire sector of data at each request thusa sector is the minimum amount of data that can be read or written at one time in generalit is desirable to keep all sectors for file together on as few tracks as possible this desire stems from two assumptions seek time is slow (it is typically the most expensive part of an / operation)and if one sector of the file is readthe next sector will probably soon be read assumption ( is called locality of referencea concept that comes up frequently in computer applications contiguous sectors are often grouped to form cluster cluster is the smallest unit of allocation for fileso all files are multiple of the cluster size the cluster size is determined by the operating system the file manager keeps track of which clusters make up each file in microsoft windows systemsthere is designated portion of the disk called the file allocation tablewhich stores information about which sectors belong to which file in contrastunix does not use clusters the smallest unit of file allocation and the smallest unit that can be read/written is sectorwhich in unix terminology is called block unix maintains information about file organization in certain disk blocks called -nodes group of physically contiguous clusters from the same file is called an extent ideallyall clusters making up file will be contiguous on the disk ( the file will consist of one extent)so as to minimize seek time required to access different portions of the file if the disk is nearly full when file is createdthere might not be an extent available that is large enough to hold the new file furthermoreif file