Ext3cow or third extended filesystem with copy-on-write is an open source, versioning file system based on the ext3 file system. Versioning is implemented through block-level copy-on-write. It shares many of its performance characteristics with ext3.

Developer(s)Zachary Peterson (ext3cow versioning), Stephen Tweedie (ext3 design and implementation), Rémy Card (original ext2 design and implementation), Theodore Ts'o (tools and improvements), Andreas Gruenbacher (xattrs and ACLs), Andreas Dilger (online resizing), et al.
Full nameThird extended file system with copy-on-write
IntroducedJuly 2003 with Linux
Partition identifier0x83 (MBR)
EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 (GPT)
Directory contentsTable, Tree
File allocationbitmap (free space), table (metadata)
Bad blocksTable
Max. volume size8TiB
Max. file size2TiB
Max. number of filesVariable1
Max. filename length255 bytes
Allowed characters in filenamesAll bytes except NUL, '/' and '@'
Dates recordedmodification (mtime), attribute modification (ctime), access (atime)
Date rangeDecember 14, 1901 - January 18, 2038
Date resolution1s
AttributesNo-atime, append-only, synchronous-write, no-dump, h-tree (directory), immutable, journal, secure-delete, top (directory), allow-undelete
File system permissionsUnix permissions, ACLs and arbitrary security attributes (Linux 2.6 and later)
Transparent compressionNo
Transparent encryptionNo (provided at the block device level)
Supported operating systemsLinux

Ext3cow provides a time-shifting interface that permits a real-time and continuous view of data in the past. Time-shifting is a novel interface, introduced in ext3cow, allowing users to navigate through and access past namespaces by adding a time component to their commands.

Ext3cow was designed to be a platform for compliance with the versioning and auditability requirements of recent US electronic record retention legislation, such as Sarbanes-Oxley and HIPAA.

A version of ext3cow for the Linux 2.6 kernel was released on March 30, 2007.

Details on ext3cow's implementation can be found in a 2005 paper.[1]

See also


  1. Zachary Peterson and Randal Burns (May 2005). "Ext3cow: A Time-Shifting File System for Regulatory Compliance" (PDF). ACM Transactions on Storage, 1(2). Archived from the original (PDF) on 2011-07-19. Retrieved 2009-02-03.
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