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ReiserFS

From Academic Kids

The ReiserFS is a general-purpose computer file system designed and implemented by a team at Namesys led by Hans Reiser, who is referred to as the project's Benevolent Dictator for Life. It is currently supported by Linux and may be included in other operating systems in the future. It also has rudimentary, unstable, unofficial support under Windows. Introduced with version 2.4.1 of the Linux kernel, it was the first journaling file system to be included in the standard kernel. ReiserFS is the default filesystem on the Slackware, SuSE, Xandros, Yoper, Linspire, FTOSX and Libranet Linux distributions.

With the exception of security updates and critical bug fixes, Namesys has ceased development on ReiserFS (now occasionally referred to as Reiser3) to concentrate on its successor, Reiser4.

Contents

Features

At the time of its introduction, ReiserFS offered features not-yet-seen in existing Linux filesystems:

  • Metadata-only journalling, its most-publicised advantage over what was the stock Linux file system at the time, ext2.
  • Online resizing (growth only), with an underlying volume manager such as LVM. Since then, Namesys has also provided tools to resize (both grow and shrink) ReiserFS filesystems offline.
  • Tail packing, a scheme to reduce internal file fragmentation. Tail packing, however, has a significant performance impact; Namesys recommends disabling the feature in performance-critical applications.

Performance

Compared to ext2 and ext3 when dealing with files under 4k, ReiserFS is often faster by a factor of 10–15. This is of great benefit in Usenet news spools, HTTP caches, mail delivery systems and other applications where small file performance is critical.

Disadvantages

ReiserFS in versions of the Linux kernel before 2.4.10 is considered unstable by Namesys and is not recommended for production use, especially in conjunction with NFS.

Some file operations (including unlink(2)) are not synchronous on ReiserFS, which can cause some subtle breakage in applications relying heavily on file-based locks.

There is no known way to defragment a ReiserFS filesystem, aside from a full dump and restore.

Early implementations of ReiserFS (prior to that in Linux 2.6.2) were also susceptible to out-of-order write hazards (files being appended to during a crash, for example, would gain a tail of garbage upon next mount). The current journaling implementation in ReiserFS, however, is now on par with that of ext3's "ordered" journaling level.

Design

ReiserFS stores file metadata ("stat items"), directory entries ("directory items"), inode block lists ("indirect items") and tails of files ("direct items") in a single, combined B+-tree keyed by a universal object id. Disk blocks allocated to nodes of the tree are "formatted internal blocks". Blocks for leaf nodes (in which items are packed end-to-end) are "formatted leaf blocks". All other blocks are "unformatted blocks" containing file contents. Directory items with too many entries or indirect items which are too long to fit into a node spill over into the right leaf neighbour. Block allocation is tracked by free space bitmaps in fixed locations.

By contrast, ext2 and other Berkeley FFS-like filesystems simply use a fixed formula for computing inode locations, hence limiting the number of files they may contain. Most such filesystems also store directories as simple lists of entries, which makes directory lookups and updates linear-time operations and degrades performance on very large directories. The single B+-tree design in ReiserFS was intended to avoid both of these problems.

See also

External links

  • Namesys (http://www.namesys.com)
  • Theodore Tso critiques (http://www.linuxmafia.com/faq/Filesystems/reiserfs.html) the ReiserFS block journaling implementation
  • convertfs (http://tzukanov.narod.ru/convertfs), a utility which performs in-place conversion between any two filesystems with sparse file support
  • An overview of the ReiserFS on-disk format (http://p-nand-q.com/download/rfstool/reiserfs_docs.html)
  • Florian Buchholz's The structure of the Reiser file system (http://www.cerias.purdue.edu/homes/florian/reiser/reiserfs.php), a very detailed description of the ReiserFS disk structures.
  • rfstool (http://p-nand-q.com/download/rfstool.html) - Limited Windows access for ReiserFS partitions.
  • ReiserDriver (http://rfsd.sourceforge.net) - A read-only file system driver to access ReiserFS partitions under Windows (currently in development).da:ReiserFS

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