File system fragmentation
In computing, file system fragmentation, sometimes called file system aging, is the inability of a file system to lay out related data sequentially (contiguously), an inherent phenomenon in storage-backed file systems that allow in-place modification of their contents. It is a special case of data fragmentation. File system fragmentation increases disk head movement or seeks, which are known to hinder throughput. The correction to existing fragmentation is to reorganize files and free space back into contiguous areas, a process called defragmentation.
When a file system is first initialized on a partition (the partition is formatted for the file system), the partition contains only a few small internal structures and is otherwise one contiguous block of empty space. This means that the allocator algorithm is completely free to place newly created files anywhere on the partition. For some time after creation, files on the file system can be laid out near-optimally. When the operating system and applications are installed or other archives are unpacked, laying out separate files sequentially also means that related files are likely to be positioned close to each other.
However, as existing files are deleted or truncated, new regions of free space are created. When existing files are appended to, it is often impossible to resume the write exactly where the file used to end, as another file may already be allocated there — thus, a new fragment has to be allocated. As time goes on, and the same factors are continuously present, free space as well as frequently appended files tend to fragment more. Shorter regions of free space also mean that the allocator is no longer able to allocate new files contiguously, and has to break them into fragments. This is especially true when the file system is more full — longer contiguous regions of free space are less likely to occur.
Note that the following is a simplification of an otherwise complicated subject. The method which is about to be explained has been the general practice for allocating files on disk and other random-access storage for over 30 years. Some operating systems do not simply allocate files one after the other, and some use various methods to try to prevent fragmentation, but in general, sooner or later, for the reasons explained in the following explanation, fragmentation will occur as time goes by on any system where files are routinely deleted or expanded. Consider the following scenario, as shown by the image on the right:
A new disk has had 5 files saved on it, named A, B, C, D and E, and each file is using 10 blocks of space (here the block size is unimportant). As the free space is contiguous the files are located one after the other (Example (1)).
If file B is deleted, a second region of 10 blocks of free space is created, and the disk becomes fragmented. The file system could defragment the disk immediately after a deletion, which would incur a severe performance penalty at unpredictable times, but in general the empty space is simply left there, marked in a table as available for later use, then used again as needed (Example (2)).
Now if a new file F requires 7 blocks of space, it can be placed into the first 7 blocks of the space formerly holding the file B, and the 3 blocks following it will remain available (Example (3)). If another new file G is added, and needs only three blocks, it could then occupy the space after F and before C (Example (4)).
If subsequently F needs to be expanded, since the space immediately following it is occupied, there are three options: (1) add a new block somewhere else and indicate that F has a second extent, (2) move files in the way of the expansion elsewhere, to allow F to remain contiguous; or (3) move file F so it can be one contiguous file of the new, larger size. The second option is probably impractical for performance reasons, as is the third when the file is very large. Indeed the third option is impossible when there is no single contiguous free space large enough to hold the new file. Thus the usual practice is simply to create an extent somewhere else and chain the new extent onto the old one (Example (5)).
Material added to the end of file F would be part of the same extent. But if there is so much material that no room is available after the last extent, then another extent would have to be created, and so on. Eventually the file system has free segments in many places and some files may be spread over many extents. Access time for those files (or for all files) may become excessively long.
To summarize, factors that typically cause or facilitate fragmentation include:
- low free space.
- frequent deletion, truncation or extension of files.
- overuse of sparse files.
Need for fragmentation
Some early file systems were unable to fragment files. One such example was the Acorn DFS file system used on the BBC Micro. Due to its inability to fragment files, the error message can't extend would at times appear, and the user would often be unable to save a file even if the disk had adequate space for it.
DFS used a very simple disk structure and files on disk were located only by their length and starting sector. This meant that all files had to exist as a continuous block of sectors and fragmentation was not possible. Using the example in the table above, the attempt to expand file F in step five would have failed on such a system with the can't extend error message. Regardless of how much free space might remain on the disk in total, it was not available to extend the data file.
Standards of error handling at the time were primitive and in any case programs squeezed into the limited memory of the BBC Micro could rarely afford to waste space attempting to handle errors gracefully. Instead, the user would find themselves dumped back at the command prompt with the Can't extend message and all the data which had yet to be appended to the file would be lost. The resulting frustration would be greater if the user had taken the trouble to check the free space on the disk beforehand and found free space. While free space on the disk may exist, the fact that it was not in the place where it was needed was not apparent without mentally analysing the numbers presented by the disk catalog and so would escape the user's notice. In addition, DFS users had almost without exception previously been accustomed to cassette file storage, which does not suffer from this error. The upgrade to a floppy disk system was expensive, but it freed users from the unreliability and terrible slowness of the cassette system, and it was a shock to make the sudden and unpleasant discovery that the upgrade might without warning cause data loss of their recent work.
File system fragmentation is projected to become more problematic with newer hardware due to the increasing disparity between sequential access speed and rotational latency (and to a lesser extent seek time), of consumer-grade hard disks, on which file systems are usually placed. Thus, fragmentation is an important problem in recent file system research and design. The containment of fragmentation not only depends on the on-disk format of the file system, but also heavily on its implementation.
In simple file system benchmarks, the fragmentation factor is often omitted, as realistic aging and fragmentation is difficult to model. Rather, for simplicity of comparison, file system benchmarks are often run on empty file systems, and unsurprisingly, the results may vary heavily from real-life access patterns.
File system fragmentation has less impact upon the performance of solid-state drives, as there is no mechanical seek time involved like with rotating media, though additional non-sequential I/O operations impacts system performance, and many file system architectures consume additional internal resources when fragmentation is present.
Types of fragmentation
File system fragmentation may occur on several levels:
- Fragmentation within individual files and their metadata.
- Free space fragmentation, making it increasingly difficult to lay out new files contiguously.
- The decrease of locality of reference between separate, but related files.
Individual file fragmentation occurs when a single file has been broken into multiple pieces (called extents on extent-based file systems). While disk file systems attempt to keep individual files contiguous, this is not often possible without significant performance penalties. File system check and defragmentation tools typically only account for file fragmentation in their "fragmentation percentage" statistic.
Free space fragmentation
Free (unallocated) space fragmentation occurs when there are several unused areas of the file system where new files or metadata can be written to. Unwanted free space fragmentation is generally caused by deletion or truncation of files, but file systems may also intentionally insert fragments ("bubbles") of free space in order to facilitate extending nearby files (see preventing fragmentation below).
File segmentation, also called related-file fragmentation, or application-level (file) fragmentation, refers to the lack of locality of reference (within the storing medium) between related files (see file sequence for more detail). Unlike the previous two types of fragmentation, file scattering is a much more vague concept, as it heavily depends on the access pattern of specific applications. This also makes objectively measuring or estimating it very difficult. However, arguably, it is the most critical type of fragmentation, as studies have found that the most frequently accessed files tend to be small compared to available disk throughput per second.
To avoid related file fragmentation and improve locality of reference (in this case called file contiguity), assumptions about the operation of applications have to be made. A very frequent assumption made is that it is worthwhile to keep smaller files within a single directory together, and lay them out in the natural file system order. While it is often a reasonable assumption, it does not always hold. For example, an application might read several different files, perhaps in different directories, in exactly the same order they were written. Thus, a file system that simply orders all writes successively, might work faster for the given application.
Techniques for mitigating fragmentation
Several techniques have been developed to fight fragmentation. They can usually be classified into two categories: preemptive and retroactive. Due to the difficulty of predicting access patterns these techniques are most often heuristic in nature and may degrade performance under unexpected workloads.
Preemptive techniques attempt to keep fragmentation at a minimum at the time data is being written on the disk. The simplest is appending data to an existing fragment in place where possible, instead of allocating new blocks to a new fragment.
Many of today's file systems attempt to preallocate longer chunks, or chunks from different free space fragments, called extents to files that are actively appended to. This largely avoids file fragmentation when several files are concurrently being appended to, thus avoiding their becoming excessively intertwined.
If the final size of a file subject to modification is known, storage for the entire file may be preallocated. For example, the Microsoft Windows swap file (page file) can be resized dynamically under normal operation, and therefore can become highly fragmented. This can be prevented by specifying a page file with the same minimum and maximum sizes, effectively preallocating the entire file.
A relatively recent technique is delayed allocation in XFS, HFS+ and ZFS; the same technique is also called allocate-on-flush in reiser4 and ext4. When the file system is being written to, file system blocks are reserved, but the locations of specific files are not laid down yet. Later, when the file system is forced to flush changes as a result of memory pressure or a transaction commit, the allocator will have much better knowledge of the files' characteristics. Most file systems with this approach try to flush files in a single directory contiguously. Assuming that multiple reads from a single directory are common, locality of reference is improved. Reiser4 also orders the layout of files according to the directory hash table, so that when files are being accessed in the natural file system order (as dictated by readdir), they are always read sequentially.
Retroactive techniques attempt to reduce fragmentation, or the negative effects of fragmentation, after it has occurred. Many file systems provide defragmentation tools, which attempt to reorder fragments of files, and sometimes also decrease their scattering (i.e. improve their contiguity, or locality of reference) by keeping either smaller files in directories, or directory trees, or even file sequences close to each other on the disk.
The now obsolete Commodore Amiga Smart File System (SFS) defragmented itself while the filesystem was in use. The defragmentation process is almost completely stateless (apart from the location it is working on), so that it can be stopped and started instantly. During defragmentation data integrity is ensured for both metadata and normal data.
Notes and references
- Some file systems, such as NTFS and ext2+, might preallocate empty contiguous regions for special purposes.
- The practice of leaving the space occupied by deleted files largely undisturbed is why undelete programs were able to work; they simply recovered the file whose name had been deleted from the directory, but whose contents were still on disk.
- http://www.8bs.com/hints/083.txt - Description of the can't extend error
- http://8bs.com/mag/1to4/basegd1.txt - Possible data loss caused by the can't extend error
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