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 CPUs and ALUs are those that are based on processor registers, address buses, or data buses of that size. A comp ...

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. There are versions for AIX,

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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 hard ...

ArcaOS ArcaOS is an operating system based on OS/2, developed and marketed by Arca Noae, LLC under license from IBM. It was codenamed Blue Lion during its development. It builds on OS/2 Warp 4.52 by adding support for new hardware, fixing defects and l ...

and

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...

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

s. The latter is available as free software under the terms of the

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(GPL).

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has another, different filesystem named JFS that is actually an OEM version of Veritas Software's VxFS. In the AIX operating system, two generations of JFS' exist, which are called ''JFS'' (''JFS1'') and ''JFS2'' respectively. 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 AIX 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 the source code, design documents, or content of the product. The open-source model is a decentralized sof ...

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

and try to fix potential

software bugs A software bug is an error, flaw or fault in the design, development, or operation of computer software that causes it to produce an incorrect or unexpected result, or to behave in unintended ways. The process of finding and correcting bugs i ...

. 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 solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It i ...

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 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 exten ...

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 in that it only journals parts of the

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 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 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. To solve this the extents are indexed in a B+ tree for better performance when locating the extent locations.

Compression

Compression is supported only in JFS1 on AIX and uses a variation of the LZ algorithm. Because of high CPU usage and increased free space

fragmentation Fragmentation or fragmented may refer to: Computers * Fragmentation (computing), a phenomenon of computer storage * File system fragmentation, the tendency of a file system to lay out the contents of files non-continuously * Fragmented distributi ...

, compression is not recommended for use other than on a single user

workstation A workstation is a special computer designed for technical or 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 systems. The term ''worksta ...

or off-line backup 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 Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine lea ...

module (since the kernel version ''2.4.18pre9-ac4'') and the complementary

userspace A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. Kernel ...

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

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 bootable computer installation including operating system which runs directly from a CD-ROM or similar storage device into a computer's memory, rather than loading f ...

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, as

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 fo ...

typically offers better performance. 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