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DMC ITSC 1405 - The Impact of Disk Fragmentation

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by Joe Kinsella While there is little dispute among IT professionals regarding the impact of disk fragmenta-tion on system performance, no independent guidelines exist to recommend the frequencyof defragmentation across an infrastructure. Some IT professionals use defragmentation asa measure of last resort, defragmenting only after system performance has sufficientlydegraded to make its impact directly noticeable to users. Others proactively schedule diskdefragmentation regularly, with the intent of eliminating the gradual accumulation of fragmented files.While just about every IT professional has his or her fragmentation horror story – about a system on which fragmentation had so severely degraded performance that it wasunusable – very few can offer more than anecdotal evidence regarding the use and impor-tance of defragmentation software.I recently decided to put fragmentation to the test, with the intent of answering twobasic questions:1. What impact does fragmentation have on user and system activities?2. How quickly does fragmentation accumulate as a result of these activities?This white paper will outline the results of the testing, draw conclusions, and make recommendations regarding managing fragmentation across your infrastructure.The Impact of Disk Fragmentation➔ContentsThe Mechanics of Disks ......................1NTFS ....................................................2The Cause of Fragmentation................3The Approach to Testing ......................3The Impact of Fragmentation ..............4Microsoft Word ................................4Microsoft Outlook ............................4Microsoft Internet Explorer ..............5Microsoft Anti-Spyware ..................5Grisoft AVG Anti-Virus Scanner............................................6The Impact of Defragmentation............6The Accumulation of Fragmentation ......................................7Underneath the Hood ..........................8Recommended Policy ..........................8Conclusions ..........................................9Copyright 2005 Diskeeper Corporation. All rights reservedThe Impact of DiskFragmentationThe Mechanics of DisksThe basic components of hard disks (see Figure 1) have not changed significantly since their invention in the 1950s. Hard disks have one ormore polished platters made of aluminum or glass that hold a magneticmedium used for storing information. The platters are stacked onto a spindle and rotated by a spindle motor at very high speeds, often in excessof 160 miles per hour. A platter has concentric circles called tracks, andeach track is divided into small sections called sectors, each capable ofholding a fixed amount of information.Small devices called heads are responsible for the actual reading andwriting of data on the platter. Each platter has two heads (for the top andbottom), and the heads are mounted on sliders positioned over the surfaceof the disks, which in turn are mounted on arms. The entire assembly isconnected to and controlled by an actuator, which in turn is connected to alogic board that allows for the communication between a computer andthe hard disk.To read or write information to the disk, an application makes a requestof an operating system to create, modify or delete a file. The operating sys-tem then translates the logical request into a physical request containingthe actual locations to be read or written on the hard disk. The logic boardthen instructs the actuator to move the heads to the appropriate track, andto read or write the appropriate sectors from the rotating platter below.The mechanical movement of the head across a platteris typically one of the most expensive operations of a harddisk. As a result, most operating systems seek to minimizethis head movement through caching, optimizing I/Orequests, and streamlining the storage of data on a disk.Streamlining the storage of data typically involves writingthe data for individual files in a file system contiguously ona platter, allowing the head to read or write data withoutneeding to be repositioned.Due to their mechanical nature, hard disks representone of the poorest-performing components in a system.Electronic components, such as the CPU, motherboard,and memory, are improving performance at a much fasterpace than hard disks, whose performance is limited by themechanics of spinning a platter and moving a head. As aresult, since an integrated system is often as fast as its slowest component, it is essential to ensure hard disks areperforming at their optimum level.NTFSBefore discussing fragmentation, let’s discuss briefly theNew Technology File System (NTFS), the file system usedwith all modern versions of Windows and the focus of thetesting for this white paper. While understanding a specificfile system is not a pre-requisite to understanding fragmen-tation, it will help clarify both the terminology used as wellas the test results.NTFS was created by Microsoft in the 1990s as part of its strategy to deliver a high-quality, high-performanceoperating system capable of competing with UNIX in a cor-porate environment. NTFS divides a hard disk into a seriesof logical clusters whose size is determined at the time thedisk is formatted with the file system. A newly formattedhard disk will by default be formatted with 4 KB clusters.The cluster size is important because it determines thesmallest unit of storage used by the file system. This meansthat a 1-byte file on a hard disk formatted with NTFS with a4K cluster size will actually physically take 4K of space onthe disk (which is why Windows reports both the Size andSize on disk for all files).The file system is divided into two parts: the Master FileTable (MFT) and a general storage area. You can think ofthe MFT as the table of contents for a hard disk. The MFTcontains a series of fixed-sized records that correspond to afile or directory stored in the general storage area. Theinformation captured in MFT records is called attributes,and includes such information as the name of the file, itssecurity descriptors, and its data. Two types of attributes2THE IMPACT OF DISK FRAGMENTATION sponsored by Diskeeper CorporationFigure 1: Overview of Hard Diskoriginal image © Seagate Technology image used with permissionCoverDiscsActuatorE BlockHead/DiscAssemblySpindleDC Power InputRead/Write HeadsBase CastingI/O ConnectorPrinted Circuit BoardFrame/BracketShock MountPrintedCircuit CableConnectorsponsored by Diskeeper Corporation THE


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DMC ITSC 1405 - The Impact of Disk Fragmentation

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