Defragmentation

In the context of administering computer systems, defragmentation is a process that reduces the amount of fragmentation in file systems. It does this by physically organizing the contents of the disk to store the pieces of each file close together and contiguously. It also attempts to create larger regions of free space using compaction to impede the return of fragmentation. Some defragmenters also try to keep smaller files within a single directory together, as they are often accessed in sequence.

Fragmentation occurs when the operating system cannot or will not allocate enough contiguous space to store a complete file as a unit, but instead puts parts of it in gaps between other files (usually those gaps exist because they formerly held a file that the operating system has subsequently deleted or because the operating system allocated excess space for the file in the first place). Larger files and greater numbers of files also contribute to fragmentation and consequent performance loss. Defragmentation attempts to alleviate these problems.

Common countermeasures

Partitioning

A common strategy to optimize defragmentation and to reduce the impact of fragmentation is to partition the hard disk(s) in a way that separates partitions of the file system that experience many more reads than writes from the more volatile zones where files are created and deleted frequently. The directories that contain the users' profiles are modified constantly (especially with the Temp directory and Internet Explorer cache creating thousands of files that are deleted in a few days). If files from user profiles are held on a dedicated partition (as is commonly done on UNIX systems), the defragmenter runs better since it does not need to deal with all the static files from other directories. For partitions with relatively little write activity, defragmentation performance greatly improves after the first defragmentation, since the defragmenter will need to defrag only a small number of new files in the future.

Immovable files

The presence of immovable system files, especially a swap file, can impede defragmentation. These files can be safely moved when the operating system is not in use. For example, ntfsresize moves these files to resize an NTFS partition, and the tool PageDefrag can defragment Windows system files such as the swap file and the files that store the Windows registry by running at boot time before the GUI is loaded. If the NTFS Master File Table must grow after the partition was formatted, fragmentation of the MFT can occur, and it cannot be safely defragmented while the partition is in use; only some defragmentation programs, such as Diskeeper or PerfectDisk (at boot time), are capable of defragmenting the MFT.

In a wide range of modern multi-user operating systems, an ordinary user cannot defragment the system disks since superuser (or "Administrator") access is required to move system files. Additionally, file systems such as NTFS (and most Unix/Linux filesystems) are designed to decrease the likelihood of fragmentation.[2][3] Improvements in modern hard drives such as RAM cache, faster platter rotation speed, and greater data density reduce the negative impact of fragmentation on system performance to some degree, though increases in commonly used data quantities offset those benefits. However, modern systems profit enormously from the huge disk capacities currently available, since partially filled disks fragment much less than full disks.[4].

Approach and defragmenters by file system type

• FAT: DOS 6.x and Windows 9x-systems come with a defragmentation utility called Defrag. The DOS version is a limited version of Norton SpeedDisk[5], and the Windows version is licensed from Diskeeper Corporation.
• NTFS: Windows 2000 and newer include a defragmentation tool based on Diskeeper. NT 4 and below do not have built-in defragmentation utilities. Unfortunately, in some cases the integrated defragger does not consolidate free space very well (particularly on the system, or "OS," drive). Thus, a heavily fragmented drive with many small files may still not have much contiguous, free space after defragmentation. So any new large file will instantly be split into small fragments with immediate impact on performance. This can happen even if the overall disk usage is less than 60%[6], but especially if the disk is rather full. It often helps to repeat defragmentation, even several times, to enhance the results. Comparing the Before and After displays each time will indicate the degree of improvement. In cases where the disk's initial free space is low, it will enhance defragmentation if large files can be moved to another partition or drive (or simply deleted, if not wanted) before defragmenting. This provides more available workspace for the defragmenter to use while running. A system drive will defragment better if it can be slaved into another computer (or if its computer can be booted from another medium), since its system files would, therefore, not be in use and free to move.
• ext2 (Linux) uses an offline defragmenter called e2defrag, which does not work with its successor ext3. Instead, a filesystem-independent defragmenter like Shake[7] may be used.
• HFS Plus (Mac OS X) In 1998 it introduced a number of optimizations to the allocation algorithms in an attempt to defragment files while they are being accessed without a separate defragmenter. If the filesystem does become fragmented, the only way to do it is to purchase a defragmenting utility, such as Coriolis System's iDefrag.