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In the maintenance of file systems, defragmentation is a process that reduces the amount of fragmentation. It does this by physically organizing the contents of the mass storage device used to store files into the smallest number of contiguous regions (fragments). It also attempts to create larger regions of free space using compaction to impede the return of fragmentation. Some defragmentation utilities try to keep smaller files within a single directory together, as they are often accessed in sequence.
Defragmentation is advantageous and relevant to file systems on electromechanical disk drives. The movement of the hard drive's read/write heads over different areas of the disk when accessing fragmented files is slower, compared to accessing the entire contents of a non-fragmented file sequentially without moving the read/write heads to seek other fragments.
Fragmentation occurs when the file 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 file 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.
Consider the following scenario, as shown by the image on the right:
An otherwise blank disk has five files, A through E, each using 10 blocks of space (for this section, a block is an allocation unit of the filesystem; the block size is set when the disk is formatted and can be any size supported by the filesystem). On a blank disk, all of these files would be allocated one after the other (see example 1 in the image). If file B were to be deleted, there would be two options: mark the space for file B as empty to be used again later, or move all the files after B so that the empty space is at the end. Since moving the files could be time consuming if there were many files which need to be moved, usually the empty space is simply left there, marked in a table as available for new files (see example 2 in the image). When a new file, F, is allocated requiring 6 blocks of space, it could be placed into the first 6 blocks of the space that formerly held file B, and the 4 blocks following it will remain available (see example 3 in the image). If another new file, G, is added and needs only 4 blocks, it could then occupy the space after F and before C (example 4 in the image). However, if file F needs to be expanded, there are three options, since the space immediately following it is no longer available:
Additionally, the concept of “fragmentation” is not only limited to individual files that have multiple extents on the disk. For instance, a group of files normally read in a particular sequence (like files accessed by a program when it is loading, which can include certain DLLs, various resource files, the audio/visual media files in a game) can be considered fragmented if they are not in sequential load-order on the disk. The disk will have to seek randomly to access them in sequence. Some groups of files may have been originally installed in the correct sequence, but drift apart with time as certain files within the group are deleted. Updates are a common cause of this, because in order to update a file, most updaters usually delete the old file first, and then write a new, updated one in its place. However, most filesystems do not write the new file in the same physical place on the disk. This allows unrelated files to fill in the empty spaces left behind. In Windows, a good defragmenter will read the Prefetch files to identify as many of these file groups as possible and place the files within them in access sequence. Another frequently good assumption is that files in any given folder are related to each other and might be accessed together.
To defragment a disk, defragmentation software (also known as a "defragmenter") can only move files around within the free space available. This is an intensive operation and cannot be performed on a filesystem with little or no free space. During defragmentation, system performance will be degraded, and it is best to leave the computer alone during the process so that the defragmenter does not get confused by unexpected changes to the filesystem. Depending on the algorithm used it may or may not be advantageous to perform multiple passes. The reorganization involved in defragmentation does not change logical location of the files (defined as their location within the directory structure).
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 web browser 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 recommended files systems, where it is typically stored in the /var partition), 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 time greatly improves after the first defragmentation, since the defragmenter will need to defragment only a small number of new files in the future.
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. The tool PageDefrag could 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. Since Windows Vista, the feature is not fully supported and has not been updated.
In NTFS, as files are added to the disk, the Master File Table (MFT) must grow to store the information for the new files. Every time the MFT cannot be extended due to some file being in the way, the MFT will gain a fragment. In early versions of Windows, it could not be safely defragmented while the partition was mounted, and so Microsoft wrote a hardblock in the defragmenting API. However, since Windows XP, an increasing number of defragmenters are now able to defragment the MFT, because the Windows defragmentation API has been improved and now supports that move operation. Even with the improvements, the first four clusters of the MFT remain unmovable by the Windows defragmentation API, resulting in the fact that some defragmenters will store the MFT in two fragments: The first four clusters wherever they were placed when the disk was formatted, and then the rest of the MFT at the beginning of the disk (or wherever the defragmenter's strategy deems to be the best place).
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 are designed to decrease the likelihood of fragmentation. Improvements in modern hard drives such as RAM cache, faster platter rotation speed, command queuing (SCSI TCQ/SATA NCQ), 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, and on a high-capacity HDD, the same partition occupies a smaller range of cylinders, resulting in faster seeks. However, the average access time can never be lower than a half rotation of the platters, and platter rotation (measured in rpm) is the speed characteristic of HDDs which has experienced the slowest growth over the decades (compared to data transfer rate and seek time), so minimizing the number of seeks remains beneficial in most storage-heavy applications. Defragmentation is just that: ensuring that there is at most one seek per file, counting only the seeks to non-adjacent tracks.
When reading data from a conventional electromechanical hard disk drive, the disk controller must first position the head, relatively slowly, to the track where a given fragment resides, and then wait while the disk platter rotates until the fragment reaches the head.
Since disks based on flash memory have no moving parts, random access of a fragment does not suffer this delay, making defragmentation to optimize access speed unnecessary. Furthermore, since flash memory can be written to only a limited number of times before it fails, defragmentation is actually detrimental (except in the mitigation of catastrophic failure).
Running most defragmenters and optimizers can cause the Microsoft Shadow Copy service to delete some of the oldest restore points, even if the defragmenters/optimizers are built on Windows API. This is due to Shadow Copy keeping track of some movements of big files performed by the defragmenters/optimizers; when the total disk space used by shadow copies would exceed a specified threshold, older restore points are deleted until the limit is not exceeded.
Besides defragmenting program files, the defragmenting tool can also reduce the time it takes to load programs and open files. For example, the Windows 9x defragmenter included the Intel Application Launch Accelerator which optimized programs on the disk. The outer tracks of a hard disk have a higher transfer rate than the inner tracks, therefore placing files on the outer tracks increases performance.