JFS2
Encyclopedia
Journaled File System or JFS is a 64-bit journaling filesystem created by IBM. Implementation for Linux kernel is available as free software under the terms of the GNU General Public License
(GPL). There are versions for AIX
, eComStation
, OS/2
and Linux
operating system
s. HP-UX
has another, different filesystem named JFS that is actually an OEM version of Veritas Software
's VxFS.
In the AIX operating system, there exist two generations of JFS filesystem that are called JFS (JFS1) and JFS2 respectively. In the 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.
version 3.1 in February, 1990. This file system, now called JFS1 on AIX, had been the premier file system for AIX over the following decade and had been installed in thousands or millions of customers' AIX systems. Historically, the JFS1 file system is very closely tied to the memory manager of AIX. This design is typical for a closed source operating system, or a file system supporting only one operating system.
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
community and work was begun to port JFS to the Linux operating system. 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.
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 shouldn't 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
and try to fix potential software bugs. He went on to add that certain distributions expect a larger resource commitment from them and opt not to support the filesystem. Nonetheless, the Backblaze
online backup runs on JFS.
file system, it was implemented from the start. The journal can be up to 128MB. JFS journals metadata only, which means that metadata will remain consistent but user files may be corrupted after a crash or power loss. JFS' journaling is similar to XFS
where it only journals parts of the inode
.
to accelerate lookups in directories. JFS can store 8 entries of a directory in the directory's inode before moving the entries to a B+ tree. JFS also indexes extents in a B+ tree.
s as necessary. Each inode is 512 Bytes. 32 Inodes are allocated on a 16KB Extent.
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
, compression is not recommended for use other than on a single user workstation
or off-line backup
areas.
module (since the kernel version 2.4.18pre9-ac4) and the complementary userspace utilities packaged under the name JFSutils (the latest version 1.1.15 released on March 4, 2011). Most Linux distributions provide support for JFS, unless it is specifically removed due to space restrictions or other concerns.
Most LiveCD distributions do not provide support of JFS because JFSutils are not installed.
According to reviews and benchmarks of the available filesystems for Linux, JFS is fast and reliable, with consistently good performance under different kinds of load, contrary to other filesystems that seem to perform better under particular usage patterns, for instance with small or large files. Another characteristic often mentioned, is that it's light and efficient with available system resources and even heavy disk activity is realized with low CPU usage.
GNU General Public License
The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
(GPL). There are versions for AIX
AIX operating system
AIX AIX AIX (Advanced Interactive eXecutive, pronounced "a i ex" is a series of proprietary Unix operating systems developed and sold by IBM for several of its computer platforms...
, eComStation
EComStation
eComStation or eCS is a PC operating system based on OS/2, published by Serenity Systems. It includes several additions and accompanying software not present in the IBM version of the system.-Differences between eComStation and OS/2:...
, OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
and Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s. HP-UX
HP-UX
HP-UX is Hewlett-Packard's proprietary implementation of the Unix operating system, based on UNIX System V and first released in 1984...
has another, different filesystem named JFS that is actually an OEM version of Veritas Software
VERITAS Software
Veritas Software Corp. was an international software company that was founded in 1983 as Tolerant Systems, renamed Veritas Software Corp. in 1989, and merged with Symantec in 2005. It was headquartered in Mountain View, California...
's VxFS.
In the AIX operating system, there exist two generations of JFS filesystem that are called JFS (JFS1) and JFS2 respectively. In the 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 AIXAIX operating system
AIX AIX AIX (Advanced Interactive eXecutive, pronounced "a i ex" is a series of proprietary Unix operating systems developed and sold by IBM for several of its computer platforms...
version 3.1 in February, 1990. This file system, now called JFS1 on AIX, had been the premier file system for AIX over the following decade and had been installed in thousands or millions of customers' AIX systems. Historically, the JFS1 file system is very closely tied to the memory manager of AIX. This design is typical for a closed source operating system, or a file system supporting only one operating system.
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
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
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
The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...
community and work was begun to port JFS to the Linux operating system. 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.
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 shouldn't 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
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
and try to fix potential software bugs. He went on to add that certain distributions expect a larger resource commitment from them and opt not to support the filesystem. Nonetheless, the Backblaze
Backblaze
Backblaze is an online backup tool that allows Windows and Mac OS X users to back up their data to an offsite data center. The service is designed for end-users, providing unlimited storage space and supporting unlimited file sizes. It allows the user to back up data continuously, manually, when...
online backup runs on JFS.
Journal
JFS is a journaling file system. Rather than adding journaling as an add-on feature like in the ext3Ext3
The ext3 or third extended filesystem is a journaled file system that is commonly used by the Linux kernel. It is the default file system for many popular Linux distributions, including Debian...
file system, it was implemented from the start. The journal can be up to 128MB. JFS journals metadata only, which means that metadata will remain consistent but user files may be corrupted after a crash or power loss. JFS' journaling is similar to XFS
XFS
XFS is a high-performance journaling file system created by Silicon Graphics, Inc. It is the default file system in IRIX releases 5.3 and onwards and later ported to the Linux kernel. XFS is particularly proficient at parallel IO due to its allocation group based design...
where it only journals parts of the inode
Inode
In computing, an inode is a data structure on a traditional Unix-style file system such as UFS. An inode stores all the information about a regular file, directory, or other file system object, except its data and name....
.
B+ Tree
JFS uses a B+ treeB+ tree
In computer science, a B+ tree or B plus tree is a type of tree which represents sorted data in a way that allows for efficient insertion, retrieval and removal of records, each of which is identified by a key. It is a dynamic, multilevel index, with maximum and minimum bounds on the number of...
to accelerate lookups in directories. JFS can store 8 entries of a directory in the directory's inode 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 inodeInode
In computing, an inode is a data structure on a traditional Unix-style file system such as UFS. An inode stores all the information about a regular file, directory, or other file system object, except its data and name....
s as necessary. Each inode is 512 Bytes. 32 Inodes are allocated on a 16KB 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
CompressionData compression
In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....
is supported only in JFS1 on AIX and uses a variation of the LZ algorithm
LZ77 and LZ78
LZ77 and LZ78 are the names for the two lossless data compression algorithms published in papers by Abraham Lempel and Jacob Ziv in 1977 and 1978. They are also known as LZ1 and LZ2 respectively. These two algorithms form the basis for most of the LZ variations including LZW, LZSS, LZMA and...
. Because of high CPU usage and increased free space fragmentation
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 , an inherent phenomenon in storage-backed file systems that allow in-place modification of their contents. It is a special case of data fragmentation...
, compression is not recommended for use other than on a single user workstation
Workstation
A workstation is a high-end microcomputer designed for technical or scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating systems...
or off-line backup
Backup
In information technology, a backup or the process of backing up is making copies of data which may be used to restore the original after a data loss event. The verb form is back up in two words, whereas the noun is 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 disc inodes for the files to be in the same AG. The second policy is to distribute unrelated data into the same AG. When there is an open file JFS will Lock the AG the file resides in and only allow the file to grow. This Reduces Fragmentation as only the open file can write to the AG.JFS 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
JFS in Linux
In the Linux operating system, JFS is supported with the kernelLinux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
module (since the kernel version 2.4.18pre9-ac4) and the complementary userspace utilities packaged under the name JFSutils (the latest version 1.1.15 released on March 4, 2011). Most Linux distributions provide support for JFS, unless it is specifically removed due to space restrictions or other concerns.
Most LiveCD distributions do not provide support of JFS because JFSutils are not installed.
According to reviews and benchmarks of the available filesystems for Linux, JFS is fast and reliable, with consistently good performance under different kinds of load, contrary to other filesystems that seem to perform better under particular usage patterns, for instance with small or large files. Another characteristic often mentioned, is that it's light and efficient with available system resources and even heavy disk activity is realized with low CPU usage.
See also
- List of file systems
- Comparison of file systemsComparison of file systems-General information:-Limits:-Metadata:-Features:-Allocation and layout policies:-Supporting operating systems:-See also:* Comparison of archive formats* Comparison of file archivers* List of archive formats* List of file archivers...
- fsckFsckThe system utility fsck is a tool for checking the consistency of a file system in Unix and Unix-like operating systems such as Linux.-Use:...
File System Check utility
External links
- JFS for Linux project website
- JFS1 File System Layout, IBM.
- JFS2 File System Layout, IBM.
- JFSRec, a console program that performs a read only extraction of files and directories from a damaged JFS filesystem