XFS is a high-performance

64-bit In computer architecture, 64-bit Integer (computer science), integers, memory addresses, or other Data (computing), data units are those that are 64 bits wide. Also, 64-bit central processing unit, CPUs and arithmetic logic unit, ALUs are those ...

journaling file system A journaling file system is a file system that keeps track of changes not yet committed to the file system's main part by recording the goal of such changes in a data structure known as a "journal", which is usually a circular log. In the even ...

created by

Silicon Graphics, Inc Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic table ...

(SGI) in 1993. It was the default file system in SGI's

IRIX IRIX ( ) is a discontinued operating system developed by Silicon Graphics (SGI) to run on the company's proprietary MIPS workstations and servers. It is based on UNIX System V with BSD extensions. In IRIX, SGI originated the XFS file system and ...

operating system starting with its version 5.3. XFS was ported to the

Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...

in 2001; as of June 2014, XFS is supported by most

Linux distribution A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading one ...

Red Hat Enterprise Linux Red Hat Enterprise Linux (RHEL) is a commercial open-source Linux distribution developed by Red Hat for the commercial market. Red Hat Enterprise Linux is released in server versions for x86-64, Power ISA, ARM64, and IBM Z and a desktop version ...

uses it as default filesystem. XFS excels in the execution of parallel

input/output In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Inputs are the signals ...

(I/O) operations due to its design, which is based on

allocation group An AG or allocation group is a subvolume in a file system which maintains its own track of free blocks and file data. This makes simultaneous file operations possible; only one write can happen to an AG at any time, but multiple operations can be ...

s (a type of subdivision of the physical volumes in which XFS is used- also shortened to ''AGs''). Because of this, XFS enables extreme scalability of I/O threads, file system bandwidth, and size of files and of the file system itself when spanning multiple physical storage devices. XFS ensures the consistency of data by employing

metadata Metadata is "data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including: * Descriptive metadata – the descriptive ...

journaling and supporting

write barrier In operating systems, write barrier is a mechanism for enforcing a particular ordering in a sequence of writes to a storage system in a computer system. For example, a write barrier in a file system is a mechanism (program logic) that ensures that ...

s. Space allocation is performed via

extents In computing, an extent is a contiguous area of storage reserved for a file in a file system, represented as a range of block numbers, or tracks on count key data devices. A file can consist of zero or more extents; one file fragment requires on ...

with data structures stored in

B+ tree A B+ tree is an m-ary tree with a variable but often large number of children per node. A B+ tree consists of a root, internal nodes and leaves. The root may be either a leaf or a node with two or more children. A B+ tree can be viewed as a B- ...

s, improving the overall performance of the file system, especially when handling large files.

Delayed allocation Allocate-on-flush (also called delayed allocation) is a file system feature implemented in HFS+, XFS, Reiser4, ZFS, Btrfs, and ext4. The feature also closely resembles an older technique that Berkeley's Unix File System, UFS called "block reallocati ...

assists in the prevention of file system fragmentation; online

defragmentation In the maintenance of file systems, defragmentation is a process that reduces the degree of fragmentation. It does this by physically organizing the contents of the mass storage device used to store files into the smallest number of contiguou ...

is also supported. A feature unique to XFS is the pre-allocation of I/O bandwidth at a pre-determined rate; this is suitable for many real-time applications. However, this feature was supported only on IRIX, and only with specialized hardware.

History

Silicon Graphics Silicon Graphics, Inc. (stylized as SiliconGraphics before 1999, later rebranded SGI, historically known as Silicon Graphics Computer Systems or SGCS) was an American high-performance computing manufacturer, producing computer hardware and soft ...

began development of XFS ("X" was meant to be filled in later but never was) in 1993. The file system was released under the

GNU General Public License The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end users the Four Freedoms (Free software), four freedoms to run, study, share, and modify the software. The license was th ...

(GPL) in May 2000. A team led by Steve Lord at SGI ported it to Linux, and first support by a

came in 2001. This support gradually became available in almost all Linux distributions. Initial support for XFS in the Linux kernel came through patches from SGI. It merged into the

Linux kernel mainline The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU oper ...

for the 2.6 series, and separately merged in February 2004 into the 2.4 series in version 2.4.25, making XFS almost universally available on Linux systems.

Gentoo Linux Gentoo Linux (pronounced ) is a Linux distribution built using the Portage package management system. Unlike a binary software distribution, the source code is compiled locally according to the user's preferences and is often optimized for the ...

became the first

to introduce an option for XFS as the default filesystem in mid-2002.

FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...

added read-only support for XFS in December 2005, and in June 2006 introduced experimental write support. However, this was intended only as an aid in migration from Linux, not as a "main" file system. FreeBSD 10 removed support for XFS. In 2009, version 5.4 of 64-bit

(RHEL) Linux distribution contained the necessary kernel support for the creation and usage of XFS file systems, but lacked the corresponding command-line tools. The tools available from

CentOS CentOS (, from Community Enterprise Operating System; also known as CentOS Linux) is a Linux distribution that provides a free and open-source community-supported computing platform, functionally compatible with its upstream source, Red Hat En ...

could operate for that purpose, and Red Hat also provided them to RHEL customers on request. RHEL 6.0, released in 2010, includes XFS support for a fee as part of Red Hat's "scalable file system add-on".

Oracle Linux Oracle Linux (abbreviated OL, formerly known as Oracle Enterprise Linux or OEL) is a Linux distribution packaged and freely distributed by Oracle, available partially under the GNU General Public License since late 2006. It is compiled from Red ...

6, released in 2011, also includes an option for using XFS. RHEL 7.0, released in June 2014, uses XFS as its default file system, including support for using XFS for the /boot partition, which previously was not practical due to bugs in the

GRUB Grub can refer to Grub (larva), of the beetle superfamily Scarabaeoidea, or as a slang term for food. It can also refer to: Places * Grub, Appenzell Ausserrhoden, Switzerland * Grub, St. Gallen, Switzerland * Grub (Amerang), a hamlet in Bavaria, ...

bootloader. Linux kernel 4.8 in August 2016 added a new feature, "reverse mapping". This is the foundation for a large set of planned features:

snapshots Snapshot, snapshots or snap shot may refer to: * Snapshot (photography), a photograph taken without preparation Computing * Snapshot (computer storage), the state of a system at a particular point in time * Snapshot (file format) or SNP, a file ...

copy-on-write Copy-on-write (COW), sometimes referred to as implicit sharing or shadowing, is a resource-management technique used in computer programming to efficiently implement a "duplicate" or "copy" operation on modifiable resources. If a resource is dupl ...

(COW) data,

data deduplication In computing, data deduplication is a technique for eliminating duplicate copies of repeating data. Successful implementation of the technique can improve storage utilization, which may in turn lower capital expenditure by reducing the overall amou ...

, reflink copies, online data and metadata scrubbing, highly accurate reporting of data loss or bad sectors, and significantly improved reconstruction of damaged or corrupted filesystems. This work required changes to XFS's on-disk format. Linux kernel 5.10, released in December 2020, introduced "bigtime", to store inode timestamps as a 64-bit nanosecond counter instead of the traditional 32-bit seconds counter. This postpones the previous

Year 2038 problem The year 2038 problem (also known as Y2038, Y2K38, or the Epochalypse) is a time formatting bug in computer systems with representing times after 03:14:07 UTC on 19 January 2038. The problem exists in systems which measure Unix time � ...

until the year 2486.

Features

Capacity

XFS is a 64-bit file system and supports a maximum file system size of 8

exbibyte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...

s minus one byte (2⁶³ − 1 bytes), but limitations imposed by the host operating system can decrease this limit.

32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32-bit units. Compared to smaller bit widths, 32-bit computers can perform large calculation ...

Linux systems limit the size of both the file and file system to 16

tebibyte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...

Journaling

In modern computing, journaling is a capability which ensures consistency of data in the file system, despite any power outages or system crash that may occur. XFS provides journaling for file system metadata, where file system updates are first written to a serial journal before the actual disk blocks are updated. The journal is a circular buffer of disk blocks that is not read in normal file system operation. The XFS journal can be stored within the data section of the file system (as an internal log), or on a separate device to minimize disk contention. In XFS, the journal primarily contains entries that describe the portions of the disk blocks changed by filesystem operations. Journal updates are performed asynchronously to avoid a decrease in performance speed. In the event of a system crash, file system operations which occurred immediately prior to the crash can be reapplied and completed as recorded in the journal, which is how data stored in XFS file systems remain consistent. Recovery is performed automatically the first time the file system is mounted after the crash. The speed of recovery is independent of the size of the file system, instead depending on the amount of file system operations to be reapplied.

Allocation groups

XFS file systems are internally partitioned into ''allocation groups'', which are equally sized linear regions within the file system. Files and directories can span allocation groups. Each allocation group manages its own

inode The inode (index node) is a data structure in a Unix-style file system that describes a file-system object such as a file or a directory. Each inode stores the attributes and disk block locations of the object's data. File-system object attribute ...

s and free space separately, providing scalability and parallelism so multiple threads and processes can perform I/O operations on the same file system simultaneously. This architecture helps to optimize parallel I/O performance on systems with multiple processors and/or cores, as metadata updates can also be parallelized. The internal partitioning provided by allocation groups can be especially beneficial when the file system spans multiple physical devices, allowing optimal usage of throughput of the underlying storage components.

Striped allocation

If an XFS file system is to be created on a striped

RAID Raid, RAID or Raids may refer to: Attack * Raid (military), a sudden attack behind the enemy's lines without the intention of holding ground * Corporate raid, a type of hostile takeover in business * Panty raid, a prankish raid by male college ...

array, a '' stripe unit'' can be specified when the file system is created. This maximizes throughput by ensuring that data allocations, inode allocations and the internal log (the journal) are aligned with the stripe unit.

Extent-based allocation

Blocks used in files stored on XFS file systems are managed with variable length

where one extent describes one or more contiguous blocks. This can shorten the list of blocks considerably, compared to file systems that list all blocks used by a file individually. Block-oriented file systems manage space allocation with one or more block-oriented bitmaps; in XFS, these structures are replaced with an extent oriented structure consisting of a pair of

s for each file system allocation group. One of the B+ trees is indexed by the length of the free extents, while the other is indexed by the starting block of the free extents. This dual indexing scheme allows for the highly efficient allocation of free extents for file system operations.

Variable block sizes

The file system block size represents the minimum allocation unit. XFS allows file systems to be created with block sizes ranging between 512 bytes and 64 KB, allowing the file system to be tuned for the expected degree of usage. When many small files are expected, a small block size would typically maximize capacity, but for a system dealing mainly with large files, a larger block size can provide a performance efficiency advantage.

Delayed allocation

XFS makes use of

lazy evaluation In programming language theory, lazy evaluation, or call-by-need, is an evaluation strategy which delays the evaluation of an expression until its value is needed (non-strict evaluation) and which also avoids repeated evaluations (sharing). The b ...

techniques for file allocation. When a file is written to the buffer cache, rather than allocating extents for the data, XFS simply reserves the appropriate number of file system blocks for the data held in memory. The actual block allocation occurs only when the data is finally flushed to disk. This improves the chance that the file will be written in a contiguous group of blocks, reducing fragmentation problems and increasing performance.

Sparse files

XFS provides a 64-bit sparse address space for each file, which allows both for very large file sizes, and for "holes" within files in which no disk space is allocated. As the file system uses an extent map for each file, the file allocation map size is kept small. Where the size of the allocation map is too large for it to be stored within the inode, the map is moved into a B+ tree which allows for rapid access to data anywhere in the 64-bit address space provided for the file.

Extended attributes

XFS provides multiple data streams for files; this is made possible by its implementation of

extended attributes Extended file attributes are file system features that enable users to associate computer files with metadata not interpreted by the filesystem, whereas regular attributes have a purpose strictly defined by the filesystem (such as permissions or ...

. These allow the storage of a number of name/value pairs attached to a file. Names are nul-terminated printable character strings which are up to 256 bytes in length, while their associated values can contain up to 64 KB of binary data. They are further subdivided into two namespaces: root and user. Extended attributes stored in the root namespace can be modified only by the superuser, while attributes in the user namespace can be modified by any user with permission to write to the file. Extended attributes can be attached to any kind of XFS inode, including symbolic links, device nodes, directories, etc. The attr utility can be used to manipulate extended attributes from the command line, and the xfsdump and xfsrestore utilities are aware of extended attributes, and will back up and restore their contents. Most other backup systems do not support working with extended attributes.

Direct I/O

For applications requiring high throughput to disk, XFS provides a direct I/O implementation that allows non-cached I/O operations to be applied directly to the userspace. Data is transferred between the buffer of the application and the disk using DMA, which allows access to the full I/O bandwidth of the underlying disk devices.

Guaranteed-rate I/O

The XFS guaranteed-rate I/O system provides an API that allows applications to reserve bandwidth to the filesystem. XFS dynamically calculates the performance available from the underlying storage devices, and will reserve bandwidth sufficient to meet the requested performance for a specified time. This is a feature unique to the XFS file system. Guaranteed rates can be "hard" or "soft", representing a trade off between reliability and performance; however, XFS will only allow "hard" guarantees if the underlying storage subsystem supports it. This facility is used mostly for real-time applications, such as video streaming. Guaranteed-rate I/O was only supported under

, and required special hardware for that purpose.

DMAPI

XFS implemented the

DMAPI Data Management API (DMAPI) is the interface defined in the X/Open document "Systems Management: Data Storage Management (XDSM) API" dated February 1997. XFS, IBM JFS, VxFS, AdvFS, StorNext and IBM Spectrum Scale file systems support DMAPI for Hie ...

interface to support

Hierarchical Storage Management Hierarchical storage management (HSM), also known as Tiered storage, is a data storage and Data management technique that automatically moves data between high-cost and low-cost storage media. HSM systems exist because high-speed storage devices, ...

in IRIX. As of October 2010, the Linux implementation of XFS supported the required on-disk metadata for DMAPI implementation, but the kernel support was reportedly not usable. For some time, SGI hosted a kernel tree which included the DMAPI hooks, but this support has not been adequately maintained, although kernel developers have stated an intention to bring this support up to date.

Snapshots

XFS does not yet provide direct support for snapshots, as it currently expects the snapshot process to be implemented by the volume manager. Taking a snapshot of an XFS filesystem involves temporarily halting I/O to the filesystem using the xfs_freeze utility, having the volume manager perform the actual snapshot, and then resuming I/O to continue with normal operations. The snapshot can then be mounted read-only for backup purposes. Releases of XFS in IRIX incorporated an integrated volume manager called XLV. This volume manager has not been ported to Linux, and XFS works with standard LVM in Linux systems instead. In recent Linux kernels, the xfs_freeze functionality is implemented in the VFS layer, and is executed automatically when the Volume Manager's snapshot functionality is invoked. This was once a valuable advantage as the

ext3 ext3, or third extended filesystem, is a journaled file system that is commonly used by the Linux kernel. It used to be the default file system for many popular Linux distributions. Stephen Tweedie first revealed that he was working on extend ...

file system could not be suspended and the volume manager was unable to create a consistent "hot" snapshot to back up a heavily busy database. Fortunately this is no longer the case. Since Linux 2.6.29, the file systems ext3,

ext4 ext4 (fourth extended filesystem) is a journaling file system for Linux, developed as the successor to ext3. ext4 was initially a series of backward-compatible extensions to ext3, many of them originally developed by Cluster File Systems for ...

GFS2 In computing, the Global File System 2 or GFS2 is a shared-disk file system for Linux computer clusters. GFS2 allows all members of a cluster to have direct concurrent access to the same shared block storage, in contrast to distributed file sys ...

and JFS have the freeze feature as well.

Online defragmentation

Although the extent-based nature of XFS and the delayed allocation strategy it uses significantly improves the file system's resistance to fragmentation problems, XFS provides a filesystem

utility (xfs_fsr, short for XFS filesystem reorganizer) that can defragment the files on a mounted and active XFS filesystem.

Online growth

XFS provides the xfs_growfs utility to perform online expansion of XFS file systems. XFS filesystems can be grown so long as there is remaining unallocated space on the device holding the filesystem. This feature is typically used in conjunction with volume management, as otherwise the

partition Partition may refer to: Computing Hardware * Disk partitioning, the division of a hard disk drive * Memory partition, a subdivision of a computer's memory, usually for use by a single job Software * Partition (database), the division of a ...

holding the filesystem will need enlarging separately.

Disadvantages

* XFS filesystems cannot () be shrunk in place, although several possible workarounds have been discussed. * Metadata operations in XFS were slower compared to journaling file systems implemented later and designed to work with much larger logs resulting in, for example, slower performance with operations such as deletions of large numbers of files. However, a new XFS feature implemented by John Nelson and called ''delayed logging'', available since version 2.6.39 of the Linux kernel mainline, is said to resolve this; performance benchmarks done by the developer in 2010 revealed performance levels to be similar to

at low thread counts, and superior at high thread counts. * Journaling cannot be disabled. However, XFS can write to an external journal on a separate block device instead.

References

External links

The XFS Linux wiki
current community wiki
XFS.org
old community wiki
crossmeta.org
community port of XFS on Windows {{File systems 1994 software Disk file systems File systems supported by the Linux kernel Formerly proprietary software IRIX