Ext3cow

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.

ext3cow

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 name	Third extended file system with copy-on-write
Introduced	July 2003; 21 years ago (2003-07) with Linux
Partition IDs	0x83 (MBR) EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 (GPT)
Structures
Directory contents	Table, Tree
File allocation	bitmap (free space), table (metadata)
Bad blocks	Table
Limits
Max volume size	8TiB
Max file size	2TiB
Max no. of files	Variable1
Max filename length	255 bytes
Allowed filename characters	All bytes except NUL, '/' and '@'
Features
Dates recorded	modification (mtime), attribute modification (ctime), access (atime)
Date range	December 14, 1901 - January 18, 2038
Date resolution	1s
Forks	Yes
Attributes	No-atime, append-only, synchronous-write, no-dump, h-tree (directory), immutable, journal, secure-delete, top (directory), allow-undelete
File system permissions	Unix permissions, ACLs and arbitrary security attributes (Linux 2.6 and later)
Transparent compression	No
Transparent encryption	No (provided at the block device level)
Other
Supported operating systems	Linux

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

• ext3
• Next3
• List of file systems
• Comparison of file systems
• Htree