Journaled File System (JFS) is a

64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit central processing units (CPU) and arithmetic logic units (ALU) are those that are based on processor registers, a ...

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 ev ...

created by

IBM International Business Machines Corporation (using the trademark IBM), nicknamed Big Blue, is an American Multinational corporation, multinational technology company headquartered in Armonk, New York, and present in over 175 countries. It is ...

. There are versions for

AIX Aix or AIX may refer to: Computing * AIX, a line of IBM computer operating systems *Alternate index, for an IBM Virtual Storage Access Method key-sequenced data set * Athens Internet Exchange, a European Internet exchange point Places Belg ...

OS/2 OS/2 is a Proprietary software, proprietary computer operating system for x86 and PowerPC based personal computers. It was created and initially developed jointly by IBM and Microsoft, under the leadership of IBM software designer Ed Iacobucci, ...

eComStation eComStation or eCS is an operating system based on OS/2 Warp for the 32-bit x86 architecture. It was originally developed by Serenity Systems and Mensys BV under license from IBM. It includes additional applications, and support for new hardwa ...

ArcaOS ArcaOS is a Proprietary software, proprietary operating system based on OS/2, developed and marketed by Arca Noae, LLC under license from IBM. It was first released in 2017 and builds on OS/2 Warp 4.52 by adding support for new hardware, fixing ...

and

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

s. The latter is available as free software under the terms of the GNU General Public License (GPL).

HP-UX HP-UX (from "Hewlett Packard Unix") is a proprietary software, proprietary implementation of the Unix operating system developed by Hewlett Packard Enterprise; current versions support HPE Integrity Servers, based on Intel's Itanium architect ...

has another, different filesystem named JFS that is actually an OEM version of

Veritas Software Veritas Technologies LLC is an American international data management company headquartered in Mountain View, California, Santa Clara, California. The company has its origins in Tolerant Systems, founded in 1983 and later renamed Veritas Softwa ...

's VxFS. In the AIX operating system, two generations of JFS exist, which are called ''JFS'' (''JFS1'') and ''JFS2'' respectively. IBM's JFS was originally designed for

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 a maximum of 32- bit units. Compared to smaller bit widths, 32-bit computers can perform la ...

systems. JFS2 was designed for

systems. In other operating systems, such as OS/2 and Linux, only the second generation exists and is called simply ''JFS''. This should not be confused with JFS in

that actually refers to JFS1.

History

IBM introduced JFS with the initial release of AIX version 3.1 in February 1990. This file system, now called ''JFS1 on AIX'', was the premier file system for AIX over the following decade and was installed in thousands or millions of customers' AIX systems. Historically, the JFS1 file system is very closely tied to the memory manager of AIX, which is a typical design for a file system supporting only one operating system. JFS was one of the first file systems to support Journaling. In 1995, work began to enhance the file system to be more scalable and to support machines that had more than one processor. Another goal was to have a more portable file system, capable of running on multiple operating systems. After several years of designing, coding, and testing, the new JFS was first shipped in OS/2 Warp Server for eBusiness in April 1999, and then in OS/2 Warp Client in October 2000. In December 1999, a snapshot of the original OS/2 JFS source was granted to the

open source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use and view the source code, design documents, or content of the product. The open source model is a decentrali ...

community and work was begun to port JFS to

. The first stable release of ''JFS for Linux'' appeared in June 2001. The ''JFS for Linux'' project is maintained by a small group of contributors known as the ''JFS Core Team''. This release of sources also worked to form the basis of a re-port back to OS/2 of the open-source JFS. In parallel with this effort, some of the JFS development team returned to the AIX Operating System Development Group in 1997 and started to move this new JFS source base to the AIX operating system. In May 2001, a second journaled file system, ''Enhanced Journaled File System (JFS2)'', was made available for AIX 5L. Early in 2008 there was speculation that IBM is no longer interested in maintaining JFS and thus it should not be used in production environments. However, Dave Kleikamp, a member of the

IBM Linux Technology Center The IBM Linux Technology Center (LTC) is an organization focused on development for the Linux kernel and related open-source software projects. In 1999, IBM created the LTC to combine its software developers interested in Linux and other open-sourc ...

and JFS Core Team, explained that they still follow changes in the

Linux kernel The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...

and try to fix potential

software bugs A software bug is a design defect ( bug) in computer software. A computer program with many or serious bugs may be described as ''buggy''. The effects of a software bug range from minor (such as a misspelled word in the user interface) to sev ...

. He went on to add that certain distributions expect a larger resource commitment from them and opt not to support the filesystem. In 2012, TRIM command support for

solid-state drive A solid-state drive (SSD) is a type of solid-state storage device that uses integrated circuits to store data persistently. It is sometimes called semiconductor storage device, solid-state device, or solid-state disk. SSDs rely on non- ...

s was added to JFS.

Features

JFS supports the following features.

Journal

JFS is a

. Rather than adding journaling as an add-on feature like in the

ext3 ext3, or third extended filesystem, is a journaling file system, journaled file system that is commonly used with the Linux kernel. It used to be the default file system for many popular Linux distributions but generally has been supplanted by ...

file system, it was implemented from the start. The journal can be up to 128 MB. JFS journals metadata only, which means that metadata will remain consistent but user files may be corrupted after a crash or power loss. JFS's journaling is similar to

XFS XFS is a high-performance 64-bit journaling file system created by Silicon Graphics, Inc (SGI) in 1993. It was the default file system in SGI's IRIX operating system starting with its version 5.3. XFS was ported to the Linux kernel in 2001; a ...

in that it only journals parts of the

inode An 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 attribu ...

B+ tree

JFS uses a

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 ...

to accelerate lookups in directories. JFS can store 8 entries of a directory in the directory's

before moving the entries to a B+ tree. JFS also indexes extents in a B+ tree.

Dynamic inode allocation

JFS dynamically allocates space for disk

s as necessary. Each inode is 512 bytes. 32 inodes are allocated on a 16 kB Extent.

Extents

JFS allocates files as an extent. An extent is a variable-length sequence of Aggregate blocks. An extent may be located in several allocation groups. To solve this the extents are indexed in a B+ tree for better performance when locating the extent locations.

Compression

Compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression * Gas compression *Compression ratio, of a ...

is supported only in JFS1 on AIX and uses a variation of the LZ algorithm. Because of high

CPU usage CPU time (or process time) is the amount of time that a central processing unit (CPU) was used for processing instructions of a computer program or operating system. CPU time is measured in clock ticks or seconds. Sometimes it is useful to con ...

and increased free space fragmentation, compression is not recommended for use other than on a single user

workstation A workstation is a special computer designed for technical or computational science, scientific applications. Intended primarily to be used by a single user, they are commonly connected to a local area network and run multi-user operating syste ...

or off-line

backup In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is "wikt:back ...

areas.

Concurrent input / output (CIO)

JFS normally applies read-shared, write-exclusive locking to files, which avoids data inconsistencies but imposes write serialization at the file level. The CIO option disables this locking. Applications such as relational databases which maintain data consistency themselves can use this option to largely eliminate filesystem overheads.

Allocation groups

JFS uses allocation groups. Allocation groups divide the aggregate space into chunks. This allows JFS to use resource allocation policies to achieve great I/O performance. The first policy is to try to cluster disk blocks and disk inodes for related data in the same AG in order to achieve good locality for the disk. The second policy is to distribute unrelated data throughout the file system in an attempt to minimize free-space fragmentation. When there is an open file JFS will lock the AG the file resides in and only allow the open file to grow. This reduces fragmentation as only the open file can write to the AG.

Superblocks

The superblock maintains information about the entire file system and includes the following fields: * Size of the file system * Number of data blocks in the file system * A flag indicating the state of the file system * Allocation group sizes * File system block size

On Linux

In the Linux operating system, JFS is supported with the kernel module (since the kernel version ''2.4.18pre9-ac4'') and the complementary

userspace A modern computer operating system usually uses virtual memory to provide separate address spaces or regions of a single address space, called user space and kernel space. This separation primarily provides memory protection and hardware prote ...

utilities packaged under the name ''JFSutils''. Most

Linux distribution A Linux distribution, often abbreviated as distro, is an operating system that includes the Linux kernel for its kernel functionality. Although the name does not imply product distribution per se, a distro—if distributed on its own—is oft ...

s support JFS unless it is specifically removed due to space restrictions, such as on

live CD A live CD (also live DVD, live disc, or live operating system) is a complete booting, bootable computer installation including operating system which runs directly from a CD-ROM or similar storage device into a computer's memory, rather than lo ...

s. According to benchmarks of the available filesystems for Linux, JFS is fast and reliable, with consistently good performance under different kinds of load. Actual usage of JFS in Linux is uncommon, however, JFS does have a niche role in Linux: it offers a case-insensitive mount option, unlike most other Linux file systems. There are also potential problems with JFS, such as its implementation of journal writes. They can be postponed until there is another trigger—potentially indefinitely, which can cause data loss over a theoretically infinite timeframe.Analysis and Evolution of Journaling File Systems - V. Prabhakaran and others
2013-06

References

External links

JFS for Linux project website

IBM

IBM
JFSRec
a console program that performs a read only extraction of files and directories from a damaged JFS filesystem {{OS/2 1990 software Compression file systems Disk file systems File systems supported by the Linux kernel Journaled File System 2 (JFS2) OS/2 technology