ReiserFS

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This article is about ReiserFS version 3. For version 4, see Reiser4.
ReiserFS
Developer Namesys
Full name ReiserFS
Introduced 2001 (Linux 2.4.1)
Partition identifier Apple_UNIX_SVR2 (Apple Partition Map)
0x83 (MBR)
EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 (GPT)
Structures
Directory contents B+ tree
File allocation Bitmap [1]
Bad blocks
Limits
Max file size 8 TiB [2]
Max number of files 232 (~4 billion) [3]
Max filename length 4032 bytes, limited to 255 by Linux VFS
Max volume size 16 TiB [4]
Allowed characters in filenames All bytes except NUL and '/'
Features
Dates recorded modification (mtime), metadata change (ctime), access (atime)
Date range December 14, 1901 - January 18, 2038
Date resolution 1s
Forks Extended attributes
Attributes
File system permissions Unix permissions, ACLs and arbitrary security attributes
Transparent compression No
Transparent encryption No
Supported operating systems Linux

ReiserFS is a general-purpose, journaled computer file system designed and implemented by a team at Namesys led by Hans Reiser. ReiserFS is currently supported on Linux. Introduced in 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 file system on the Elive, Xandros, Yoper, Linspire, GoboLinux and Kurumin Linux[citation needed] Linux distributions. ReiserFS was the default file system in Novell's SUSE Linux Enterprise until Novell decided to move to ext3 on October 12, 2006 for future releases.[1] Although the change was rumored to be a result of principal author Hans Reiser being charged with the murder of his wife two days earlier, SUSE stated that the timing of the announcement was coincidental and unrelated.[2]

Namesys considers ReiserFS (now occasionally referred to as Reiser3) stable and feature-complete and, with the exception of security updates and critical bug fixes, has thus ceased development on it to concentrate on its successor, Reiser4.

Contents

Features

At the time of its introduction, ReiserFS offered features that had not been available in existing Linux file systems:

  • Metadata-only journaling (also block journaling, since Linux 2.6.8), its most-publicized 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 file systems offline.
  • Tail packing, a scheme to reduce internal fragmentation. Tail packing, however, can have a significant performance impact. Reiser4 may have improved this by packing tails where it doesn't hurt performance.[3]

Performance

Compared with ext2 and ext3 in version 2.4 of the Linux kernel, when dealing with files under 4 KiB and with tail packing enabled, ReiserFS is often faster by a factor of 10–15.[citation needed] This is of great benefit in Usenet news spools, HTTP caches, mail delivery systems and other applications where performance with small files is critical.

However, because ReiserFS still uses the Big Kernel Lock (BKL) - a global kernel-wide lock - in some places, it does not scale very well for systems with multiple cores, as the critical code parts are only ever executed by one core at a time.[2]

Criticism

Some directory operations (including unlink(2)) are not synchronous on ReiserFS, which can result in data corruption with applications relying heavily on file-based locks (such as mail transfer agents qmail[4] and Postfix[5]) if the machine halts before it has synchronized the disk.[6]

There are no programs to specifically defragment a ReiserFS file system, although tools have been written to automatically copy the contents of fragmented files hoping that more contiguous blocks of free space can be found. However, Reiser4 will have a repacker that optimizes file fragmentation.[7]

fsck

The tree rebuild process of ReiserFS's fsck has attracted much criticism: If the file system becomes so badly corrupt that its internal tree is unusable, performing a tree rebuild operation may further corrupt existing files or introduce new entries with unexpected contents.[8] But this action is not part of normal operation or a normal file system check and has to be explicitly initiated and confirmed by the administrator.

Nevertheless it is recommended not to store ReiserFS v3 images on a ReiserFS v3 partition (e.g. backups or disk images for emulators) without transforming them (e.g., by compressing or encrypting) in order to avoid misleading the filesystem. Reformatting an existing ReiserFS v3 partition can also leave behind data that could confuse the rebuild operation and make files from the old system reappear. This also allows malicious users to intentionally store files that will confuse the rebuilder. As the metadata is always in a consistent state after a file system check, corruption here means that contents of files are merged in unexpected ways with the contained file system's metadata. The ReiserFS successor, Reiser4, fixes this problem.

Earlier issues

ReiserFS in versions of the Linux kernel before 2.4.16 were considered unstable by Namesys and not recommended for production use, especially in conjunction with NFS.[9]

Early implementations of ReiserFS (prior to that in Linux 2.6.2) were also susceptible to out-of-order write hazards. For example, files being appended to during a crash gained a tail of garbage upon next mount.[citation needed] But the current journaling implementation in ReiserFS 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 file systems simply use a fixed formula for computing inode locations, hence limiting the number of files they may contain.[10] Most such file systems 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 avoids both of these problems due to better scalability properties.

See also

Notes and references

  1. ^ Shankland, Stephen. "Novell makes file storage software shift", Business Tech, c, 2006-10-12. 
  2. ^ a b Mahoney, Jeff. "Announcement was unrelated to Reiser's legal troubles", 2006-10-04. 
  3. ^ Reiser, Hans. Reiser4 is released!. Retrieved on 2006-07-15.
  4. ^ Daniel Robbins (2001), "Advanced file system implementor's guide". Retrieved 5. July 2006
  5. ^ Matthias Andree (2001), LKML post on Postfix synchronity assumptions. Retrieved 15. July 2006
  6. ^ NEOHAPSIS - Peace of Mind Through Integrity and Insight
  7. ^ Hans Reiser, Reiser4 design, repacker. Retrieved 5. July 2006
  8. ^ Theodore Ts'o LKML post. Retrieved 5. July 2006
  9. ^ ReiserFS download page, see warning. Retrieved 5. July 2006
  10. ^ Mingming Cao, Theodore Y. Ts'o, Badari Pulavarty, Suparna Bhattacharya (2005-07-26). "State of the Art: Where we are with the Ext3 file system". 2005 Linux Symposium, Ottawa, Canada: IBM Linux Technology Center. Retrieved on 2007-03-08. 

External links