JFS is a port of IBM's Journaling Filesystem to Linux. JFS was originally developed for IBM's OS/2 operating system and later adapted for use as the enterprise filesystem used on its pSeries/AIX-based systems. IBM released the source code for JFS to the open source community in 2000 and has actively participated in the continuing development and support of this filesystem for Linux since that time.

JFS is similar to ReiserFS in that it uses binary trees to store information about files. JFS is heavily based on transactions, in much the same way that databases are, using them as the basis for the records that it maintains in its journal. JFS provides a very fast method of data allocation based on extents. An extent is a contiguous series of data blocks that can be allocated, read, written, and managed at one time.

JFS also makes clever use of filesystem data structures such as the inode (information node) data structure that is associated with each single file or directory in the filesystem. At least one inode exists for every file in the filesystem, but JFS creates them only when files and directories are created. In traditional filesystems, the number of inodes (and thus the number of files) on a filesystem was dictated at filesystem creation time. This could lead to a situation in which no more files could be created because there was nowhere to store information about the file. Creating inodes as files and directories means that a JFS filesystem can contain an essentially unlimited number of files and allows a JFS filesystem to be scalable in the traditional sense. As JFS is a 64-bit filesystem, it is also able to allocate space for extremely large files, unlike existing 32-bit filesystems that can create files only up to 4GB in size because of addressing issues.

JFS is available as an option on openSUSE, but it has not been a supported filesystem for SLES since the release of SLES 10. More information about JFS is available on the IBM site at www.ibm.com/developerworks/library/l-jfs.html.

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