JFS (file system)
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| JFS | |
|---|---|
| Developer | IBM et al. |
| Full name | IBM Journaled File System |
| Introduced | 1990 and 1999 (JFS1 in AIX 3.1 and JFS in OS/2 4.5) |
| Partition identifier | 0x35 (MBR) |
| Structures | |
| Directory contents | B+ Trees |
| File allocation | Bitmap/Extents |
| Bad blocks | |
| Limits | |
| Max file size | 4 PiB |
| Max number of files | No limit defined |
| Max filename length | 255 bytes |
| Max volume size | 32 PiB |
| Allowed characters in filenames | Any Unicode except NUL |
| Features | |
| Dates recorded | Modification (mtime), Attribute modification (ctime), Access (atime) |
| Date range | |
| Date resolution | 1ns |
| Forks | Yes |
| Attributes | Yes |
| File system permissions | POSIX, ACLs |
| Transparent compression | Only in JFS1 on AIX |
| Transparent encryption | No (provided at the block device level) |
| Supported operating systems | AIX, OS/2, Linux |
Journaled File System or JFS is a 64-bit journaling filesystem created by IBM. It is available under the GNU GPL. There are versions for AIX, eComStation, OS/2 and Linux operating systems. HP-UX has another, different filesystem named JFS that is actually an OEM version of Veritas Software's VxFS.
In AIX operating system, there exist two generations of JFS filesystem that are called JFS (JFS1) and JFS2 respectively.[1] [2] In the other operating systems, such as OS/2 and Linux, only the second generation exists and is called simply JFS.[3] This should not be confused with JFS in AIX that actually refers to JFS1.
Contents |
[edit] History
IBM introduced JFS with the initial release of AIX 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.[1] 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 Linux operating system. The first stable release of JFS for Linux appeared in June, 2001.[3]
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.[1] [3]
Around the summer of 2007 there was speculation that IBM is no longer interested in maintaining JFS and thus it shouldn't be used in production environments. [4] However early in 2008, a member of the IBM Linux Technology Center 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. [5]
[edit] Features
Other than a journal, JFS has more features which make it more robust.[6][7]
[edit] B+ Tree
A B+ tree is used to accelerate lookups in larger directories. The B+ tree is also used for the addressing structure with extent descriptors.
[edit] Dynamic Inode Allocation
JFS dynamically allocates space for disk inodes as necessary. Each inode is 512 Bytes. 32 Inodes are allocated on a 16KB Extent.
[edit] 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 to solve this the extents are indexed in a B+ tree for better performance when locating the extent locations.
[edit] Compression
Compression is supported only in JFS1 on AIX and uses a variation of 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.[2] [8]
[edit] 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.[9].
[edit] 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.
[edit] 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
[edit] JFS in 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 utilities packaged under the name JFSutils (the latest version 1.1.12 released in August, 2007). Most Linux distributions provide support for JFS, unless it is specifically removed due to space restrictions or other concerns.
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. [10] [11]
[edit] See also
- List of file systems
- Comparison of file systems
- fsck File System Check utility
[edit] References
- ^ a b c A mini-FAQ for JFS. JFS for Linux project.
- ^ a b Comparison of JFS1 and JFS2 on AIX. IBM.
- ^ a b c Interview With the People Behind JFS, ReiserFS & XFS.
- ^ Re: which to use: ext3, JFS, XFS, ReiserFS?
- ^ SourceForge.net: jfs-discussion
- ^ JFS overview. Steve Best, IBM. Retrieved on 2008-01-09.
- ^ JFS Layout. Steve Best, IBM. Retrieved on 2008-05-01.
- ^ AIX Wiki: JFS. IBM.
- ^ Improving Database Performance With AIX Concurrent I/O - White Paper
- ^ Filesystems (ext3, reiser, xfs, jfs) comparison on Debian Etch
- ^ http://fsbench.netnation.com/
