Encyclopedia of Computer Science and Technology

hashing 223third party) defragmentation utility, users can periodicallyreorganize their hard drive so that files are again stored incontiguous sectors.Files can also be reorganized to optimize space ratherthan access time. If an operating system has a minimumcluster size c4K, a single file with only 32 bytes of datawill still consume 4,096 bytes. However, if all the files arewritten together as one huge file (with an index that specifieswhere each file begins) that waste of space would beavoided. This is the principle of disk compression. Diskcompression does slow access somewhat (due to the needto look up and position to the actual data location for afile) and the system becomes more fragile (since garblingthe giant file would prevent access to the data in perhapsthousands of originally separate files). The low cost ofhigh capacity drives today has made compression lessnecessary.Interfacing Hard DrivesWhen the operating system wants to read or write datato the disk, it must send commands to the driver, a programthat translates high-level commands to the instructionsneeded to operate the disk controller, which in turnoperates the motors controlling the disk heads. The twomost commonly used interfaces for PC internal hard drivestoday are both based on the ATA (Advanced TechnologyAttachment) standard. The older standard is PATA (parallelATA), also called IDE (Integrated Drive Electronics)or EIDE (Enhanced IDE). Increasingly common today isSATA, or serial ATA. Another alternative, more commonlyused on servers, is SCSI (Small Computer System Interface).SCSI is more expensive but has several advantages: It hasthe ability to organize incoming commands for greater efficiencyand also features greater flexibility (an EIDE controllercan connect only two hard drives, while SCSI can “daisychain” a large number of disk drives or other peripherals).In practice, the two interfaces perform about equally well.USB (Universal Serial Bus) is frequently used to interfacewith external hard drive units (see usb).The capacity continues to increase, with data able tobe written more densely or perhaps in multiple layers onthe same disk surface. Denser storage also offers the abilityto make drives more compact. Already hard drives with adiameter of about an inch have been built by IBM and othersfor use in digital cameras.The proliferation of multimedia (including video) andthe growth of databases has fed a voracious appetite forhard drive space. Disks with a capacity of 1 TB (terabyte,or trillion bytes) were starting to come onto the market by2007. For larger installations, disk arrays (see raid) offerhigh capacity and data-protecting redundancy.Perpendicular hard drive recording technology recentlydeveloped by Hitachi aligns the magnetic “grains” that holdbits of data vertically instead of horizontally, allowing fora considerably higher data density (and thus capacity, for agiven size disk). Hitachi suggests that eventually 1 TB canbe stored on a 3.5" disk.Drive speeds (and thus data throughput) have also beenincreasing, with more users choosing 7200 rpm rather thanthe formerly standard 5400 rpm drives. (There are drives asfast as 15,000 rpm, but for most applications the benefits ofhigher speed drop off rapidly.)Another factor in data access time and throughput is theuse of a dedicated memory device (see cache) to “pre-fetch”data likely to be needed. Windows Vista allows memoryfrom some USB memory sticks (see flash drive) to workas a disk cache. “Hybrid” hard drives directly integratingRAM and drive storage are also available.Further ReadingJacob, Bruce, Spencer Ng, and David Wang. Memory Systems:Cache, DRAM, Disk. San Francisco: Morgan Kaufmann, 2007.“Perpendicular Hard Drive Recording Technology.” Available online.URL: http://www.webopedia.com/DidYouKnow/Computer_Science/2006/perpendicular_hard_drive_technology.asp.Accessed August 6, 2007.“Storage.” Tom’s Hardware Guide. Available online. URL: http://www.tomshardware.com/storage/index.html. Accessed August 6,2007.“What’s Inside a Hard Drive?” Available online. URL: http://www.webopedia.com/DidYouKnow/Hardware_Software/2002/InsideHardDrive.aspdYouKnow/Hardware_Software/2002/InsideHardDrive.asp. Accessed August 6, 2007.hashingA hash is a numeric value generated by applying a mathematicalformula to the numeric values of the charactersin a string of text (see characters and strings). The formulais chosen so that the values it produces are always thesame length (regardless of the length of the original text)and are very likely to be unique. (Two different stringsshould not produce the same hash value. Such an event iscalled a collision.)ApplicationsThe two major application areas for hashing are informationretrieval and cryptographic certification. In databases,an index table can be built that contains the hash values forthe key fields and the corresponding record number for eachfield, with the entries in hash value order. To search thedatabase, an input key is hashed and the value is comparedwith the index table (which can be done using a very fastbinary search). If the hash value is found, the correspondingrecord number is used to look up the record. This tends tobe much faster than searching an index file directly.Alternatively, a “coarser” but faster hashing function canbe used that will give the same hash value to small groups(called bins) of similar records. In this case the hash fromthe search key is matched to a bin and then the recordswithin the bin are searched for an exact match.In cryptography an encrypted message can be hashed,producing a unique fixed-length value. (The fixed lengthprevents attackers from using mathematical relationshipsthat might be discoverable from the field lengths.) Thehashed message can then be encrypted again to create anelectronic signature (see certificate, digital). For longmessages this is more efficient than having to apply the signaturefunction to each block of the encrypted message, yet