Misplaced Pages

#886113

79-460: Btrfs (pronounced as "better F S", "butter F S", "b-tree F S", or B.T.R.F.S.) is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (distinct from Linux's LVM ), developed together. It was created by Chris Mason in 2007 for use in Linux , and since November 2013, the file system's on-disk format has been declared stable in

158-415: A USENIX conference in 2007. Mason, an engineer working on ReiserFS for SUSE at the time, joined Oracle later that year and began work on a new file system based on these B-trees. In 2008, the principal developer of the ext3 and ext4 file systems, Theodore Ts'o , stated that although ext4 has improved features, it is not a major advance; it uses old technology and is a stop-gap. Ts'o said that Btrfs

237-495: A hierarchy . Brown notes that Plan 9 from Bell Labs , the intended successor to Unix, does not include the concept of a hard link. Windows NT 3.1 and later support hard links on the NTFS file system. Windows 2000 introduces a CreateHardLink() function to create hard links, but only for files, not directories. The DeleteFile() function can remove them. To create a hard link on Windows, end-users can use: To interrogate

316-447: A binary diff between them (by using the btrfs send command) that can be replayed later (by using btrfs receive ), possibly on a different Btrfs file system. The send–receive feature effectively creates (and applies) a set of data modifications required for converting one subvolume into another. The send/receive feature can be used with regularly scheduled snapshots for implementing a simple form of file system replication , or for

395-561: A circular directory structure, rather than a branching structure like a tree . For that reason, some file systems forbid the creation of additional hard links to directories. POSIX -compliant operating systems , such as Linux , Android , macOS , and the Windows NT family , support multiple hard links to the same file, depending on the file system. For instance, NTFS and ReFS support hard links, while FAT does not. Let two hard links, named "LINK A.TXT" and "LINK B.TXT", point to

474-424: A device, device type, directory prefix, file path separator, or file type. File systems typically support organizing files into directories , also called folders , which segregate files into groups. This may be implemented by associating the file name with an index in a table of contents or an inode in a Unix-like file system. Directory structures may be flat (i.e. linear), or allow hierarchies by allowing

553-672: A directory to contain directories, called subdirectories. The first file system to support arbitrary hierarchies of directories was used in the Multics operating system. The native file systems of Unix-like systems also support arbitrary directory hierarchies, as do, Apple 's Hierarchical File System and its successor HFS+ in classic Mac OS , the FAT file system in MS-DOS 2.0 and later versions of MS-DOS and in Microsoft Windows ,

632-531: A file for its hard links, end-users can use: The Windows Component Store uses hard links to keep track of different versions of components stored on the hard disk drive. On Unix-like systems, the link() system call can create additional hard links to existing files. To create hard links, end-users can use: To interrogate a file for its hard links, end-users can use: Unix-like emulation or compatibility software running on Microsoft Windows, such as Cygwin and Subsystem for UNIX-based Applications , allow

711-469: A file system can be managed by the user via various utility programs. Hard links In computing , a hard link is a directory entry (in a directory -based file system ) that associates a name with a file . Thus, each file must have at least one hard link. Creating additional hard links for a file makes the contents of that file accessible via additional paths (i.e., via different names or in different directories). This causes an alias effect :

790-443: A file system creates a file, it allocates space for data. Some file systems permit or require specifying an initial space allocation and subsequent incremental allocations as the file grows. To delete a file, the file system records that the file's space is free; available to use for another file. A local file system manages storage space to provide a level of reliability and efficiency. Generally, it allocates storage device space in

869-422: A file system implementation. The physical file system layer provides relatively low-level access to a storage device (e.g. disk). It reads and writes data blocks , provides buffering and other memory management and controls placement of blocks in specific locations on the storage medium. This layer uses device drivers or channel I/O to drive the storage device. A file name , or filename , identifies

SECTION 10

#1732784055887

948-417: A file to consuming applications and in some cases users. A file name is unique so that an application can refer to exactly one file for a particular name. If the file system supports directories, then generally file name uniqueness is enforced within the context of each directory. In other words, a storage can contain multiple files with the same name, but not in the same directory. Most file systems restrict

1027-706: A forked file system on the Macintosh, and Microsoft supports streams in NTFS. Some file systems maintain multiple past revisions of a file under a single file name; the file name by itself retrieves the most recent version, while prior saved version can be accessed using a special naming convention such as "filename;4" or "filename(-4)" to access the version four saves ago. See comparison of file systems#Metadata for details on which file systems support which kinds of metadata. A local file system tracks which areas of storage belong to which file and which are not being used. When

1106-477: A granular manner, usually multiple physical units (i.e. bytes ). For example, in Apple DOS of the early 1980s, 256-byte sectors on 140 kilobyte floppy disk used a track/sector map . The granular nature results in unused space, sometimes called slack space , for each file except for those that have the rare size that is a multiple of the granular allocation. For a 512-byte allocation, the average unused space

1185-511: A large directory would thus generate many more disk seeks between non-adjacent files—a notable performance drain in other file systems with hash-ordered directories such as ReiserFS , ext3 (with Htree-indexes enabled) and ext4, all of which have TEA -hashed filenames. To avoid this, each directory entry has a directory index item , whose key value of the item is set to a per-directory counter that increments with each new directory entry. Iteration over these index items thus returns entries in roughly

1264-493: A new Btrfs, an existing Btrfs can be used as a read-only "seed" file system. The new file system will then act as a copy-on-write overlay on the seed, as a form of union mounting . The seed can be later detached from the Btrfs, at which point the rebalancer will simply copy over any seed data still referenced by the new file system before detaching. Mason has suggested this may be useful for a Live CD installer, which might boot from

1343-494: A per-filename header of 4 bytes, this would be less than 350.) Applications which made heavy use of multiple same-directory hard links, such as git , GNUS , GMame and BackupPC were observed to fail at this limit. The limit was eventually removed (and as of October 2012 has been merged pending release in Linux 3.7) by introducing spillover extended reference items to hold hard link filenames which do not otherwise fit. File data

1422-419: A process can open the file by any one of its paths and change its content. By contrast, a soft link or “shortcut” to a file is not a direct link to the data itself, but rather a reference to a hard link or another soft link. Every directory is itself a special file on many systems, containing a list of file names instead of other data. Hence, multiple hard links to directories are possible, which could create

1501-457: A read-only Btrfs seed on an optical disc, rebalance itself to the target partition on the install disk in the background while the user continues to work, then eject the disc to complete the installation without rebooting. In his 2009 interview, Mason stated that support for encryption was planned for Btrfs. In the meantime, a workaround for combining encryption with Btrfs is to use a full-disk encryption mechanism such as dm-crypt  / LUKS on

1580-400: A snapshot-capable file system that would use this data structure almost exclusively—not just for metadata and file data, but also recursively to track space allocation of the trees themselves. This allowed all traversal and modifications to be funneled through a single code path, against which features such as copy on write, checksumming and mirroring needed to be implemented only once to benefit

1659-431: A stream of bytes . Typically, to read file data, a program provides a memory buffer and the file system retrieves data from the medium and then writes the data to the buffer. A write involves the program providing a buffer of bytes that the file system reads and then stores to the medium. Some file systems, or layers on top of a file system, allow a program to define a record so that a program can read and write data as

SECTION 20

#1732784055887

1738-585: A structure; not an unorganized sequence of bytes. If a fixed length record definition is used, then locating the n record can be calculated mathematically, which is relatively fast compared to parsing the data for record separators. An identification for each record, also known as a key, allows a program to read, write and update records without regard to their location in storage. Such storage requires managing blocks of media, usually separating key blocks and data blocks. Efficient algorithms can be developed with pyramid structures for locating records. Typically,

1817-411: A subvolume is not possible until all subvolumes below it in the nesting hierarchy are deleted; as a result, top-level subvolumes cannot be deleted. Any Btrfs file system always has a default subvolume, which is initially set to be the top-level subvolume, and is mounted by default if no subvolume selection option is passed to mount . The default subvolume can be changed as required. A Btrfs snapshot

1896-432: A user's use of storage space. A file system typically ensures that stored data remains consistent in both normal operations as well as exceptional situations like: Recovery from exceptional situations may include updating metadata, directory entries and handling data that was buffered but not written to storage media. A file system might record events to allow analysis of issues such as: Many file systems access data as

1975-405: Is 256 bytes. For 64 KB clusters, the average unused space is 32 KB. Generally, the allocation unit size is set when the storage is configured. Choosing a relatively small size compared to the files stored, results in excessive access overhead. Choosing a relatively large size results in excessive unused space. Choosing an allocation size based on the average size of files expected to be in

2054-457: Is a subvolume that shares its data (and metadata) with some other subvolume, using Btrfs' copy-on-write capabilities, and modifications to a snapshot are not visible in the original subvolume. Once a writable snapshot is made, it can be treated as an alternate version of the original file system. For example, to roll back to a snapshot, a modified original subvolume needs to be unmounted and the snapshot needs to be mounted in its place. At that point,

2133-534: Is invisible to the end user and the system still works correctly. However this can degrade performance on some storage hardware that work better with contiguous blocks such as hard disk drives . Other hardware such as solid-state drives are not affected by fragmentation. A file system often supports access control of data that it manages. The intent of access control is often to prevent certain users from reading or modifying certain files. Access control can also restrict access by program in order to ensure that data

2212-430: Is kept outside the tree in extents , which are contiguous runs of disk data blocks. Extent blocks default to 4 KiB in size, do not have headers and contain only (possibly compressed) file data. In compressed extents, individual blocks are not compressed separately; rather, the compression stream spans the entire extent. Files have extent data items to track the extents which hold their contents. The item's key value

2291-437: Is mapped to an empty directory of the same name inside the snapshot. Taking snapshots of a directory is not possible, as only subvolumes can have snapshots. However, there is a workaround that involves reflinks spread across subvolumes: a new subvolume is created, containing cross-subvolume reflinks to the content of the targeted directory. Having that available, a snapshot of this new volume can be created. A subvolume in Btrfs

2370-510: Is modified in a controlled way. Examples include passwords stored in the metadata of the file or elsewhere and file permissions in the form of permission bits, access control lists , or capabilities . The need for file system utilities to be able to access the data at the media level to reorganize the structures and provide efficient backup usually means that these are only effective for polite users but are not effective against intruders. Methods for encrypting file data are sometimes included in

2449-440: Is no guarantee that the target volume's file system is compatible with hard linking.) The maximum number of hard links to a single file is limited by the size of the reference counter. On Unix-like systems the counter is 4,294,967,295 (on 32-bit machines) or 18,446,744,073,709,551,615 (on 64-bit machines). In some file systems, the number of hard links is limited more strictly by their on-disk format. For example, as of Linux 3.11,

Btrfs - Misplaced Pages Continue

2528-419: Is quite different from a traditional Logical Volume Manager (LVM) logical volume. With LVM, a logical volume is a separate block device , while a Btrfs subvolume is not and it cannot be treated or used that way. Making dd or LVM snapshots of btrfs leads to data loss if either the original or the copy is mounted while both are on the same computer. Given any pair of subvolumes (or snapshots), Btrfs can generate

2607-550: Is stored inside the allocation group itself. Additional attributes can be associated on file systems, such as NTFS , XFS , ext2 , ext3 , some versions of UFS , and HFS+ , using extended file attributes . Some file systems provide for user defined attributes such as the author of the document, the character encoding of a document or the size of an image. Some file systems allow for different data collections to be associated with one file name. These separate collections may be referred to as streams or forks . Apple has long used

2686-569: Is the better direction because "it offers improvements in scalability, reliability, and ease of management". Btrfs also has "a number of the same design ideas that reiser3 / 4 had". Btrfs 1.0, with finalized on-disk format, was originally slated for a late-2008 release, and was finally accepted into the Linux kernel mainline in 2009. Several Linux distributions began offering Btrfs as an experimental choice of root file system during installation. In July 2011, Btrfs automatic defragmentation and scrubbing features were merged into version 3.0 of

2765-465: Is the starting byte offset of the extent. This makes for efficient seeks in large files with many extents, because the correct extent for any given file offset can be computed with just one tree lookup. Snapshots and cloned files share extents. When a small part of a large such extent is overwritten, the resulting copy-on-write may create three new extents: a small one containing the overwritten data, and two large ones with unmodified data on either side of

2844-438: The mount(8) utility. It can also be accessed by mounting the top-level subvolume, in which case subvolumes are visible and accessible as its subdirectories. Subvolumes can be created at any place within the file system hierarchy, and they can also be nested. Nested subvolumes appear as subdirectories within their parent subvolumes, similarly to the way a top-level subvolume presents its subvolumes as subdirectories. Deleting

2923-480: The btrfs qgroup command) into hierarchies to implement quota pools. Quota groups only apply to subvolumes and snapshots, while having quotas enforced on individual subdirectories, users, or user groups is not possible. However, workarounds are possible by using different subvolumes for all users or user groups that require a quota to be enforced. As the result of having very little metadata anchored in fixed locations, Btrfs can warp to fit unusual spatial layouts of

3002-402: The commit mount option. Ohad Rodeh's original proposal at USENIX 2007 noted that B+ trees , which are widely used as on-disk data structures for databases, could not efficiently allow copy-on-write-based snapshots because its leaf nodes were linked together: if a leaf was copied on write, its siblings and parents would have to be as well, as would their siblings and parents and so on until

3081-633: The Linux kernel mainline . Besides Mason at Oracle, Miao Xie at Fujitsu contributed performance improvements. In June 2012, Mason left Oracle for Fusion-io , which he left a year later with Josef Bacik to join Facebook . While at both companies, Mason continued his work on Btrfs. In 2012, two Linux distributions moved Btrfs from experimental to production or supported status: Oracle Linux in March, followed by SUSE Linux Enterprise in August. In 2015, Btrfs

3160-572: The NTFS file system in the Windows NT family of operating systems, and the ODS-2 (On-Disk Structure-2) and higher levels of the Files-11 file system in OpenVMS . In addition to data, the file content, a file system also manages associated metadata which may include but is not limited to: A file system stores associated metadata separate from the content of the file. Most file systems store

3239-437: The ext4 file system limits the number of hard links on a file to 65,000. Windows limits enforces a limit of 1024 hard links to a file on NTFS volumes. On Linux Weekly News , Neil Brown criticized hard links as high-maintenance, since they complicate the design of programs that handle directory trees, including archivers and disk usage tools. These apps must take care to de-duplicate files that are linked multiple times in

Btrfs - Misplaced Pages Continue

3318-536: The " .. " file (parent directory) consistent, operating systems do not generally allow hard links to directories. UNIX System V allowed them, but only the superuser had permission to make such links. Mac OS X v10.5 (Leopard) and newer use hard links on directories for the Time Machine backup mechanism only. Hard links can be created to files only on the same volume, i.e., within the same file system. (Different volumes may have different file systems. There

3397-615: The Linux kernel . Btrfs is intended to address the lack of pooling , snapshots , checksums , and integral multi-device spanning in Linux file systems . Mason, the principal Btrfs author, stated that its goal was "to let [Linux] scale for the storage that will be available. Scaling is not just about addressing the storage but also means being able to administer and to manage it with a clean interface that lets people see what's being used and makes it more reliable". The core data structure of Btrfs‍—‌the copy-on-write B-tree ‍—‌was originally proposed by IBM researcher Ohad Rodeh at

3476-404: The associated inode are preserved as long as a single hard link (directory reference) points to it or any process keeps the associated file open. On POSIX -compliant operating systems, the reference count for a file or directory is returned by the stat() or fstat() system calls in the st_nlink field of struct stat . To prevent loops in the filesystem, and to keep the interpretation of

3555-415: The backend storage devices. The btrfs-convert tool exploits this ability to do an in-place conversion of an ext2/3/4 or ReiserFS file system, by nesting the equivalent Btrfs metadata in its unallocated space—while preserving an unmodified copy of the original file system. The conversion involves creating a copy of the whole ext2/3/4 metadata, while the Btrfs files simply point to the same blocks used by

3634-429: The broken filesystem itself (i.e., non-destructively). In normal use, Btrfs is mostly self-healing and can recover from broken root trees at mount time, thanks to making periodic data flushes to permanent storage, by default every 30 seconds. Thus, isolated errors will cause a maximum of 30 seconds of filesystem changes to be lost at the next mount. This period can be changed by specifying a desired value (in seconds) with

3713-464: The btrfs documentation suggests that its --repair option be used only if you have been advised by "a developer or an experienced user". As of August 2022, the SLE documentation recommends using a Live CD, performing a backup and only using the repair option as a last resort. There is another tool, named btrfs-restore , that can be used to recover files from an unmountable filesystem, without modifying

3792-421: The computer main memory can be set up as a RAM disk that serves as a storage device for a file system. File systems such as tmpfs can store files in virtual memory . A virtual file system provides access to files that are either computed on request, called virtual files (see procfs and sysfs ), or are mapping into another, backing storage. From c.  1900 and before the advent of computers

3871-519: The concepts. The logical file system layer provides relatively high-level access via an application programming interface (API) for file operations including open, close, read and write – delegating operations to lower layers. This layer manages open file table entries and per-process file descriptors. It provides file access, directory operations, security and protection. The virtual file system , an optional layer, supports multiple concurrent instances of physical file systems, each of which called

3950-448: The conversion can be undone at any time (although doing so will erase any changes made after the conversion to Btrfs). All converted files are available and writable in the default subvolume of the Btrfs. A sparse file holding all of the references to the original ext2/3/4 filesystem is created in a separate subvolume, which is mountable on its own as a read-only disk image, allowing both original and converted file systems to be accessed at

4029-505: The entire file system. Btrfs is structured as several layers of such trees, all using the same B-tree implementation. The trees store generic items sorted by a 136-bit key. The most significant 64 bits of the key are a unique object id . The middle eight bits are an item type field: its use is hardwired into code as an item filter in tree lookups. Objects can have multiple items of multiple types. The remaining (least significant) 64 bits are used in type-specific ways. Therefore, items for

SECTION 50

#1732784055887

4108-496: The entire tree was copied. He suggested instead a modified B-tree (which has no leaf linkage), with a refcount associated to each tree node but stored in an ad hoc free map structure and certain relaxations to the tree's balancing algorithms to make them copy-on-write friendly. The result would be a data structure suitable for a high-performance object store that could perform copy-on-write snapshots, while maintaining good concurrency . At Oracle later that year, Mason began work on

4187-413: The ext2/3/4 files. This makes the bulk of the blocks shared between the two filesystems before the conversion becomes permanent. Thanks to the copy-on-write nature of Btrfs, the original versions of the file data blocks are preserved during all file modifications. Until the conversion becomes permanent, only the blocks that were marked as free in ext2/3/4 are used to hold new Btrfs modifications, meaning that

4266-468: The file around. However, some editors, such as GNU Emacs , break the hard link concept. When opening a file for editing, e.g., "LINK B.TXT", emacs renames "LINK B.TXT" to "LINK B.TXT~", loads "LINK B.TXT~" into the editor, and saves the modified contents to a newly created "LINK B.TXT". Now, "LINK A.TXT" and "LINK B.TXT" no longer shares the same data. (This behavior can be changed using the emacs variable backup-by-copying .) Any number of hard links to

4345-403: The file once occupied. Most file systems that support hard links use reference counting . The system stores an integer value with each logical data section that represents the total number of hard links that have been created to point to the data. When a new link is created, this value is increased by one. When a link is removed, the value is decreased by one. When the counter becomes zero,

4424-418: The file system does not create a new link pointing to an existing inode ; instead, it creates a new inode that initially shares the same disk blocks with the original file. As a result, cloning works only within the boundaries of the same Btrfs file system, but since version 3.6 of the Linux kernel it may cross the boundaries of subvolumes under certain circumstances. The actual data blocks are not duplicated; at

4503-416: The file system. This is very effective since there is no need for file system utilities to know the encryption seed to effectively manage the data. The risks of relying on encryption include the fact that an attacker can copy the data and use brute force to decrypt the data. Additionally, losing the seed means losing the data. Some operating systems allow a system administrator to enable disk quotas to limit

4582-500: The length of a file name. Some file systems match file names as case sensitive and others as case insensitive. For example, the names MYFILE and myfile match the same file for case insensitive, but different files for case sensitive. Most modern file systems allow a file name to contain a wide range of characters from the Unicode character set. Some restrict characters such as those used to indicate special attributes such as

4661-595: The names of all the files in one directory in one place—the directory table for that directory—which is often stored like any other file. Many file systems put only some of the metadata for a file in the directory table, and the rest of the metadata for that file in a completely separate structure, such as the inode . Most file systems also store metadata not associated with any one particular file. Such metadata includes information about unused regions— free space bitmap , block availability map —and information about bad sectors . Often such information about an allocation group

4740-493: The operating system frees the logical data section. (The OS may not to do so immediately, e.g., when there are outstanding file handles open, for performance reasons, or to enable the undelete command.) This is a simple method for the file system to track the use of a given area of storage, as zero values indicate free space and nonzero values indicate used space. The maintenance of this value guarantees that there will be no dangling hard links pointing nowhere. The data section and

4819-425: The original subvolume may also be deleted. The copy-on-write (CoW) nature of Btrfs means that snapshots are quickly created, while initially consuming very little disk space. Since a snapshot is a subvolume, creating nested snapshots is also possible. Taking snapshots of a subvolume is not a recursive process; thus, if a snapshot of a subvolume is created, every subvolume or snapshot that the subvolume already contains

SECTION 60

#1732784055887

4898-511: The overwrite. To avoid having to re-write unmodified data, the copy-on-write may instead create bookend extents , or extents which are simply slices of existing extents. Extent data items allow for this by including an offset into the extent they are tracking: items for bookends are those with non-zero offsets. The extent allocation tree acts as an allocation map for the file system. Unlike other trees, items in this tree do not have object ids. They represent regions of space: their key values hold

4977-414: The physical data may be created. To access the data, a user only needs to specify the name of any existing link; the operating system will resolve the location of the actual data. Even if the user deletes one of the hard links, the data is still accessible through any other link that remains. Once the user deletes all of the links, if no process has the file open, the operating system frees the disk space that

5056-507: The purpose of performing incremental backups . A quota group (or qgroup ) imposes an upper limit to the space a subvolume or snapshot may consume. A new snapshot initially consumes no quota because its data is shared with its parent, but thereafter incurs a charge for new files and copy-on-write operations on existing files. When quotas are active, a quota group is automatically created with each new subvolume or snapshot. These initial quota groups are building blocks which can be grouped (with

5135-409: The same object end up adjacent to each other in the tree, grouped by type. By choosing certain key values, objects can further put items of the same type in a particular order. Interior tree nodes are simply flat lists of key-pointer pairs, where the pointer is the logical block number of a child node. Leaf nodes contain item keys packed into the front of the node and item data packed into the end, with

5214-471: The same order as stored on disk. Files with hard links in multiple directories have multiple reference items, one for each parent directory. Files with multiple hard links in the same directory pack all of the links' filenames into the same reference item. This was a design flaw that limited the number of same-directory hard links to however many could fit in a single tree block. (On the default block size of 4 KiB, an average filename length of 8 bytes and

5293-406: The same physical data. A text editor opens "LINK A.TXT", modifies it and saves it. When the editor (or any other app) opens "LINK B.TXT", it can see those changes made to "LINK A.TXT", since both file names point to the same data. So from a user's point of view this is one file with several filenames. Editing any filename modifies "all" files, however deleting "any" filename except the last one keeps

5372-480: The same time, due to the copy-on-write (CoW) nature of Btrfs, modifications to any of the cloned files are not visible in the original file and vice versa. Cloning should not be confused with hard links , which are directory entries that associate multiple file names with a single file. While hard links can be taken as different names for the same file, cloning in Btrfs provides independent files that initially share all their disk blocks. Support for this Btrfs feature

5451-499: The same time. Deleting this sparse file frees up the space and makes the conversion permanent. In 4.x versions of the mainline Linux kernel, the in-place ext3/4 conversion was considered untested and rarely used. However, the feature was rewritten from scratch in 2016 for btrfs-progs 4.6. and has been considered stable since then. In-place conversion from ReiserFS was introduced in September 2017 with kernel 4.13. When creating

5530-406: The size of the file system, whereas those in Btrfs are dynamic and created as needed.) Each block group is associated with a block group item . Inode items in the file system tree include a reference to their current block group. File system A file system provides a data storage service that allows applications to share mass storage . Without a file system, applications could access

5609-610: The starting offsets and lengths of the regions they represent. The file system divides its allocated space into block groups which are variable-sized allocation regions that alternate between preferring metadata extents (tree nodes) and data extents (file contents). The default ratio of data to metadata block groups is 1:2. They are intended to use concepts of the Orlov block allocator to allocate related files together and resist fragmentation by leaving free space between groups. (Ext3 block groups, however, have fixed locations computed from

5688-488: The storage in incompatible ways that lead to resource contention , data corruption and data loss . There are many file system designs and implementations – with various structure and features and various resulting characteristics such as speed, flexibility, security, size and more. Files systems have been developed for many types of storage devices , including hard disk drives (HDDs), solid-state drives (SSDs), magnetic tapes and optical discs . A portion of

5767-429: The storage tends to minimize unusable space. As a file system creates, modifies and deletes files, the underlying storage representation may become fragmented . Files and the unused space between files will occupy allocation blocks that are not contiguous. A file becomes fragmented if space needed to store its content cannot be allocated in contiguous blocks. Free space becomes fragmented when files are deleted. This

5846-442: The terms file system , filing system and system for filing were used to describe methods of organizing, storing and retrieving paper documents. By 1961, the term file system was being applied to computerized filing alongside the original meaning. By 1964, it was in general use. A local file system's architecture can be described as layers of abstraction even though a particular file system design may not actually separate

5925-537: The two growing toward each other as the leaf fills up. Within each directory, directory entries appear as directory items , whose least significant bits of key values are a CRC32C hash of their filename. Their data is a location key , or the key of the inode item it points to. Directory items together can thus act as an index for path-to-inode lookups, but are not used for iteration because they are sorted by their hash, effectively randomly permuting them. This means user applications iterating over and opening files in

6004-414: The underlying devices and to create the Btrfs filesystem on top of that layer. As of 2020, the developers were working to add keyed hash like HMAC ( SHA256 ). Unix systems traditionally rely on " fsck " programs to check and repair filesystems. This functionality is implemented via the btrfs check program. Since version 4.0 this functionality is deemed relatively stable. However, as of December 2022,

6083-502: Was added in version 7.5 of the GNU coreutils , via the --reflink option to the cp command. In addition to data cloning ( FICLONE ), Btrfs also supports out-of-band deduplication via FIDEDUPERANGE . This functionality allows two files with (even partially) identical data to share storage. A Btrfs subvolume can be thought of as a separate POSIX file namespace , mountable separately by passing subvol or subvolid options to

6162-594: Was adopted as the default filesystem for SUSE Linux Enterprise Server (SLE) 12. In August 2017, Red Hat announced in the release notes for Red Hat Enterprise Linux (RHEL) 7.4 that it no longer planned to move Btrfs to a fully supported feature (it's been included as a "technology preview" since RHEL 6 beta) noting that it would remain available in the RHEL 7 release series. Btrfs was removed from RHEL 8 in May 2019. RHEL moved from ext4 in RHEL 6 to XFS in RHEL 7. In 2020, Btrfs

6241-399: Was selected as the default file system for Fedora 33 for desktop variants. As of version 5.0 of the Linux kernel, Btrfs implements the following features: Btrfs provides a clone operation that atomically creates a copy-on-write snapshot of a file . Such cloned files are sometimes referred to as reflinks , in light of the proposed associated Linux kernel system call . By cloning,

#886113