|Directory contents||B+ tree|
|Max. volume size||16 TiB|
|Max. file size||1 EiB (8 TiB on 32 bit systems)|
|Max. number of files||232-3 (~4 billion)|
|Max. filename length||4032 bytes, limited to 255 by Linux VFS|
|Allowed characters in filenames||All bytes except NUL and '/'|
|Dates recorded||Modification (mtime), metadata change (ctime), access (atime)|
|Date range||December 14, 1901 - January 18, 2038|
|Date resolution||1 s|
|File system permissions||Unix permissions, ACLs and arbitrary security attributes|
|Supported operating systems||Linux, ReactOS|
ReiserFS is a general-purpose, journaling file system initially designed and implemented by a team at Namesys led by Hans Reiser. ReiserFS is currently supported on Linux (without quota support) licensed as GPLv2. 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 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.
Namesys considered ReiserFS version 3.6 which introduced a new on-disk format allowing bigger filesizes, now occasionally referred to as Reiser3, as stable and feature-complete and, with the exception of security updates and critical bug fixes, ceased development on it to concentrate on its successor, Reiser4. Namesys went out of business in 2008 after Reiser's conviction for murder. The product is now maintained as open source by volunteers. The reiserfsprogs 3.6.27 were released on 25 July 2017.
At the time of its introduction, ReiserFS offered features that had not been available in existing Linux file systems. One example is tail packing--a scheme to reduce internal fragmentation. Tail packing can have a significant performance impact. Reiser4 may have improved this by packing tails where it does not negatively affect performance.
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 of that time simply used a fixed formula for computing inode locations, hence limiting the number of files they may contain. 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.
Before Linux 2.6.33, ReiserFS heavily used the big kernel lock (BKL)--a global kernel-wide lock--which does not scale well for systems with multiple cores, as the critical code parts are only ever executed by one core at a time.
Jeff Mahoney of SUSE wrote a post on 14 September 2006 proposing to move from ReiserFS to ext3 for the default installation file system. Some reasons he mentioned were scalability, "performance problems with extended attributes and ACLs", "a small and shrinking development community", and that "Reiser4 is not an incremental update and requires a reformat, which is unreasonable for most people." On October 4 he wrote a response comment on a blog in order to clear up some issues. He wrote that his proposal for the switch was unrelated to Hans Reiser being under trial for murder.[failed verification] Mahoney wrote he "was concerned that people would make a connection where none existed" and that "the timing is entirely coincidental and the motivation is unrelated."
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 and Postfix) if the machine halts before it has synchronized the disk.
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, a "repacker" tool was planned for the next Reiser4 file system to deal with file fragmentation. With the rise of Solid State Disks this problem became irrelevant.
The tree rebuild process of ReiserFS's fsck has attracted much criticism by the *nix community: If the file system becomes so badly corrupted that its internal tree is unusable, performing a tree rebuild operation may further corrupt existing files or introduce new entries with unexpected contents, 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.
ReiserFS v3 images should not be stored 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 confusing the rebuild. 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.
Early implementations of ReiserFS (prior to that in Linux 2.6.2) were also susceptible to out-of-order write hazards. But the current journaling implementation in ReiserFS is now on par with that of ext3's "ordered" journaling level.