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Programming technique for efficiently duplicating data
Copy-on-write (COW), also called implicit sharing or shadowing, is a resource-management technique used in programming to manage shared data efficiently
Copy-on-write
Copy-on-write file system
") is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (distinct from Linux's
Btrfs
Additional storage that enables faster access to main storage
data stored in a cache might be the result of an earlier computation or a copy of data stored elsewhere. A cache hit occurs when the requested data can
Cache_(computing)
Technique in object-oriented programming
references can cause problems. Lazy copy is related to copy-on-write. In C++, a lazy copy occurs by default when invoking the copy constructor. The following presents
Object_copying
Computer security vulnerability
Dirty COW (Dirty copy-on-write) is a computer security vulnerability of the Linux kernel that affected all Linux-based operating systems, including Android
Dirty_COW
Character strings and handling in C++
"Hello, world!"s; A copy-on-write implementation means string a = "hello!"; string b = a; // Copy constructor does not actually copy the content of a to
C++_string_handling
File system for tracking pending changes
copy of every block that will later be written to the main file system. If there is a crash when the main file system is being written to, the write can
Journaling_file_system
Object whose state cannot be modified after it is created
directly in almost all modern hardware, is copy-on-write (COW). Using this technique, when a user asks the system to copy an object, it instead merely creates
Immutable_object
Topics referred to by the same term
journalists and authors, ready for copy editing and typesetting The output of copywriters, who are employed to write material which encourages consumers
Copy
Writing text for the purpose of advertising or marketing
Copywriting is aimed at selling products or services. The output, called copy or sales copy, is written content that aims to increase brand awareness and ultimately
Copywriting
Copy-on-write file system
every RAID-Z write being a full-stripe write. This, when combined with the copy-on-write transactional semantics of ZFS, eliminates the write hole error
ZFS
Data structure that always preserves the previous version of itself when it is modified
performance characteristics for m modifications of an array of size n. Copy-on-write memory management can reduce the price for an update from Θ ( n ) {\displaystyle
Persistent_data_structure
Open-source file system
several techniques found in other file systems. NOVA uses log structure, copy-on-write (COW), journaling, and log-structured metadata updates to provide strong
NOVA_(filesystem)
Journaling file system for Linux
include the bitmap and the inode table of other block groups. Btrfs – Copy-on-write file system Comparison of file systems Extended file attributes – Set
Ext4
Operating system virtualization paradigm
their own copies.) This is easier to back up, more space-efficient and simpler to cache than the block-level copy-on-write schemes common on whole-system
OS-level_virtualization
Phenomenon associated with solid state storage
Son, Yongseok (October 2024). "A Survey on Copy-on-Write File Systems". 2024 15th International Conference on Information and Communication Technology
Write_amplification
Software library providing an embedded transactional key-value database
shared across multiple processes or threads using shared memory with copy-on-write semantics (known historically as a single-level store). Most former
Lightning Memory-Mapped Database
Lightning_Memory-Mapped_Database
Journaling file system for IRIX and Linux
a large set of planned features: snapshots, copy-on-write (COW) data, data deduplication, reflink copies, online data and metadata scrubbing, highly accurate
XFS
Copy-on-write file system for Linux
Bcachefs is a copy-on-write (COW) file system for Linux-based operating systems. It is intended to compete with the modern features of ZFS or Btrfs. It
Bcachefs
Copy-on-write file system developed by Apple
the drive's read/write heads must physically seek out scattered data fragments. Besides that, a key feature of APFS is "copy-on-write," which allows for
Apple_File_System
(BFS) - file system used by z/VM for Unix applications Btrfs – is a copy-on-write file system for Linux announced by Oracle in 2007 and published under
List_of_file_systems
Recorded state of a computer storage system at a particular point in time
be also described as performing additional copy-on-write operations triggered by the writes to snapshots. On Linux, Logical Volume Manager (LVM) allows
Snapshot_(computer_storage)
In computing, an operation whereby a process creates a copy of itself
exact copy of all the memory segments of the parent process. In modern UNIX variants that follow the virtual memory model from SunOS-4.0, copy-on-write semantics
Fork_(system_call)
Duplication of information or an artifact
digital copying is website copy, digital copying has more interpretation than just the basic concept of disk read and write itself. Digital Copy is a sample
Copying
Copy-on-write file system
this approach called Redirect-on-write (ROW) technique. ROW is much faster on rewrite operations compare to Copy-on-write where old data block going to
Write_Anywhere_File_Layout
Microsoft technology for storage snapshots
Wikibooks has a book on the topic of: Guide to Windows Commands List of Microsoft Windows components Snapshot (computer storage) Copy-on-write "Volume Snapshot
Shadow_Copy
Operating system microkernel
to the port concept, using the copy-on-write (COW) mechanism provided by the virtual memory system. Instead of copying data between programs, all that
Mach_(kernel)
Improving the formatting, style, and accuracy of text
working on hard copy write their corrections in the text directly, leaving the margins for querying. Usually, the copy editor is asked to write in a bright
Copy_editing
Written material
and advertising. In advertising, the term "copy" means the output of copywriters, who are employed to write material which encourages consumers to buy
Copy_(publishing)
Command in Linux
attribute functionality, including the C flag, which turns off the built-in copy-on-write (CoW) feature of btrfs due to slower performance associated with CoW
Chattr
Computer storage space allocation method
implement snapshots by applying copy-on-write to each LE. In this scheme, the volume manager will copy the LE to a copy-on-write table just before it is written
Logical_volume_management
Filesystem service for Linux
as writable, but without actually allowing writes to change the file system, also known as copy-on-write. This may be desirable when the media is physically
UnionFS
Constructor that copies the state of another object
same data between several objects, thus saving space. The copy-on-write strategy makes a copy of the data only when it is written to. Reference counting
Copy_constructor_(C++)
Software engineering object-oriented API
turtles") It is possible to create immutable fluent interfaces that utilise copy-on-write semantics. In this variation of the pattern, instead of modifying internal
Fluent_interface
Virtual hard drive format
used by QEMU, a hosted virtual machine monitor. It stands for "QEMU Copy On Write" and uses a disk storage optimization strategy that delays allocation
Qcow
making write intent logs unnecessary. Bcachefs is a transactional filesystem using copy-on-write semantics, guaranteeing an always-consistent on-disk state
Comparison_of_file_systems
Computer operations that do not copy memory
zero-copy through at least these system calls: sendfile; write, writev + mmap when writing data to a network socket. MacOS should support zero-copy through
Zero-copy
Clustered file system
simpler. Oracle announced version 1.6 in November 2010 which included a copy on write feature called reflink. Free and open-source software portal GlusterFS
OCFS2
Storage area in computer file system
extent-based file systems do allocate-on-flush. Many modern fault-tolerant file systems also do copy-on-write, although that increases fragmentation
Extent_(file_systems)
Proprietary file system developed by Microsoft
committed. It uses similar techniques as those used for Volume Shadow Copies (i.e. copy-on-write) to ensure that overwritten data can be safely rolled back, and
NTFS
Type of denial-of-service software attack
own copy of the same infinite loop. This has the effect of causing an exponential growth in processes. As modern Unix systems generally use a copy-on-write
Fork_bomb
Synchronization mechanism
in a manner that enforces read/write consistency and enables read/write concurrency. Concurrency control Copy-on-write Lock (computer science) Lock-free
Read-copy-update
Software framework of Linux kernel
snapshot-origin – used for creation of LVM snapshots, as part of the underlying copy-on-write scheme striped – stripes the data across physical devices, with the
Device_mapper
Logical volume management system
read-only snapshots of logical volumes (LVM1), leveraging a copy on write (CoW) feature, or read/write snapshots (LVM2) VGs can be split or merged in situ as
Logical Volume Manager (Linux)
Logical_Volume_Manager_(Linux)
Data storage technology
ensures that each block is its own stripe, so every block is complete. Copy-on-write (COW) transactional semantics guard metadata associated with stripes
RAID
File system feature
When used in conjunction with copy-on-write as it is in ZFS, it can convert slow random writes into fast sequential writes. "About Disk Optimization with
Allocate-on-flush
Topics referred to by the same term
collected without breaking vacuum Cell on wheels, a means of providing temporary mobile phone network coverage Copy-on-write, in computing Al-Baqara, the second
Cow_(disambiguation)
Data processing technique to eliminate duplicate copies of repeating data
individual entries have a copy-on-write behavior that is non-aliasing, i.e. changing one copy afterwards will not affect other copies. Microsoft's ReFS also
Data_deduplication
of physical storage (probably on a hard disk), typically of the order of 1 to 64 KiB. Shadow paging is a copy-on-write technique for avoiding in-place
Shadow_paging
Act of creating a new computer file with content matching an exiting file
provides for file copying via commands cp, copy and similar variants. mv also copies files but only when the source and destination are on different file
File_copying
Stored data in computer systems
filesystems that presents a copy of the filesystem as if it were frozen at a specific point in time, often by a copy-on-write mechanism. Snapshotting a
Backup
Proprietary file system developed by Microsoft
and redundancy, integration of RAID functionality, a switch to copy/allocate on write for data and metadata updates, handling of very long paths and filenames
ReFS
Apple's general-purpose, open-source, compiled programming language
applications as well. Array, Dictionary, and Set all utilize copy on write so that their data are copied only if and when the program attempts to change a value
Swift_(programming_language)
Interface to device driver that appears in filesystem
the hardware device. They do not necessarily allow programs to read or write single characters at a time; that is up to the device in question. The character
Device_file
Open-source implementation of ZFS file system
implementation supports features like data compression, data deduplication, copy-on-write clones, snapshots, RAID-Z, and virtual devices that can create filesystems
OpenZFS
SAV – Virtual Machine saved state COW – Copy-on-write QCOW – QEMU copy-on-write QCOW2 – QEMU copy-on-write – version 2 QED – QEMU enhanced disk format
List_of_file_formats
Collections in Java
making a copy of the object during iteration, similar to how CopyOnWriteArrayList acts as the concurrent replacement for a synchronized List. On the other
Java_collections_framework
Data segmentation technique
have other advantages that are not directly related to data striping (copy-on-write, etc). Partition alignment Link aggregation "Red Hat Enterprise Linux
Data_striping
OS-level virtualisation
security breaches. To save space on such installations, each virtual server's file system can be created as a tree of copy-on-write hard links to a "template"
Linux-VServer
Journaling filesystem based on ext3
available through a special namespace. An advantage of copy-on-write is that when Next3 writes new data, the blocks containing the old data can be retained
Next3
Computer security vulnerability (2026)
perform controlled 4‑byte writes into the page cache, which backs the in‑memory copies of files. By replacing code in the in‑memory copy of a readable executable
Copy_Fail
Cache coherence protocol for computer processors
A write may only be performed freely if the cache line is in the Modified or Exclusive state. If it is in the Shared state, all other cached copies must
MESI_protocol
File system designed to run on flash memory
systems is: when the flash store is to be updated, the file system will write a new copy of the changed data over to a fresh block, remap the file pointers
Flash_file_system
State of computer where no additional memory can be allocated
the end-user has disabled them. The condition may arise because of copy-on-write after fork().[further explanation needed] The kernels of operating systems
Out_of_memory
Linux file system
emphasizes snapshots. The log-structured approach is a specific form of copy-on-write behavior, so it naturally lends itself to the creation of file system
NILFS
two ways: Invalidate all the copies on other caches ("broadcast-invalidate") Update all the copies on other caches (write-broadcasting) to the new correct
List of cache coherency protocols
List_of_cache_coherency_protocols
Flash memory file system
compression using LZO or LZ4 (with Linux 5.6), or zstd (with Linux 5.7) Copy-on-write F2FS divides the whole volume into a number of segments, each of which
F2FS
Log-structured file system for flash memory devices
to use large write buffers. There is no practical way to tell how much usable free space is left on a device since this depends both on how well additional
JFFS2
Linux storage configuration daemon
Linux portal Comparison of file systems List of file systems APFS – a copy-on-write file system for macOS, iOS, tvOS, and watchOS HAMMER – DragonFly BSD's
Stratis (configuration daemon)
Stratis_(configuration_daemon)
File format for virtual hard disk drive containers
image can also refer to a parent image and only store changes made in a copy-on-write fashion. This enables creating a snapshot of a virtual machine's state
VMDK
System tool for checking the consistency of a file system
design is reflected by the -F flag used at boot. ZFS and Btrfs, two full copy-on-write file systems. They avoid in-place changes to assure levels of consistency
Fsck
Equivalence of all cached copies of a memory location
which processor P2 changes the value of S in its own cached copy to 20. If we ensure only write propagation, then P3 and P4 will certainly see the changes
Cache_coherence
Characteristic of state of a computer system that outlives the process that created it
data Persistent data structure Persistent identifier Persistent memory Copy-on-write CRUD Java Data Objects Java Persistence API System prevalence Orthogonality
Persistence (computer science)
Persistence_(computer_science)
Software-based, block-level cache of drive data stored in the host computer's main memory
pages can be written to as program code is often mapped as read-only or copy-on-write; in the latter case, modifications to code will only be visible to the
Page_cache
Design flaw in 1993-1997 Intel processors
fact are, since this is the same basic mechanism used for implementing copy-on-write. Additional workarounds other than the official ones from Intel have
Pentium_F00F_bug
Computer data storage architecture that manages data as objects
point-in-time copy of all the objects in a partition into a new partition. The OSD can implement a space-efficient copy using copy-on-write techniques so
Object_storage
Executable file format
intermodular calls, it minimizes the number of memory pages that require copy-on-write changes, thus conserving memory and disk I/O. If a call is known to
Portable_Executable
Kernel feature to share memory pages between multiple processes or virtualised guests
extra physical pages for re-use. It also marks both virtual pages as "copy-on-write" (COW), so that kernel will automatically remap a virtual page back
Kernel_same-page_merging
Computing technique
Combining on Intel Pentium Pro/2/3/4 CPUs in Windows 9x and DOS CTU (Internet Archive cached copy) enable LFB and Banked VGA Write Combining on AMD K6 CPUs
Write_combining
Unix command to securely delete files and devices
manual and automatic backups, file system snapshots, copy-on-write filesystems, wear leveling on flash drives, caching such as NFS caching, and journaling
Shred_(Unix)
Computer memory that can be accessed by multiple processes
mechanism known as copy-on-write that transparently copies the page when a write is attempted, and then lets the write succeed on the private copy. Compared to
Shared_memory
Operating system kernel and predecessor to Mach
changed by one program or another would have to be physically copied, using the copy-on-write algorithm. To understand the difference, consider two interacting
Accent_kernel
Type of computer memory
divided into four DDR4-like bank groups Power-saving improvements: Data-Copy and Write-X (all one or all zero) commands to decrease data transfer Dynamic frequency
LPDDR
Protection of software, films, music, other media
Copy protection, also known as content protection, copy prevention and copy restriction, is any measure to enforce copyright by preventing the reproduction
Copy_protection
Relational database service
2017. In August 2017, Aurora Fast Cloning (copy-on-write) feature was added allowing customers to create copies of their databases. In May 2018, Aurora Backtrack
Amazon_Aurora
Virtualization software
on virtual machines. Ability to use host X server to run graphical application (wireshark...). Copy-on-write file systems, allowing to economize on disk
Marionnet
Word processing software
pfs:Write is a word processor released by Software Publishing Corporation (SPC) in 1983 for IBM PC compatibles running MS-DOS and the Apple II. It includes
Pfs:Write
Computer error correction technique
operations, as well as general RAID statuses, are always available. As a copy-on-write (CoW) file system for Linux, Btrfs provides fault isolation, corruption
Data_scrubbing
Early word processing shareware
the number of lines of text that would fit on a page. PC-Write's implementation of free-form editing can copy and paste a block of text anywhere. For instance
PC-Write
every RAID-Z write being a full-stripe write. This, when combined with the copy-on-write transactional semantics of ZFS, eliminates the write hole error
Non-standard_RAID_levels
Computer memory management scheme
and data). When programs want to modify the library's code, they use copy-on-write, so memory is only allocated when needed. Shared memory is an efficient
Memory_paging
Journaling file system for Linux
partially occurring) Different transaction models: journaling, write-anywhere (copy-on-write), hybrid transaction model More efficient journaling through
Reiser4
Computer file system
versioning filesystem. Backup Comparison of version-control software Copy-on-write Object storage Daniel G. Bobrow, Jerry D. Burchfiel, Daniel L. Murphy
Versioning_file_system
source novel, a book that has sold 15 million copies and has been credited with inspiring Kurt Cobain to write the Nirvana classic Scentless Apprentice."
List_of_best-selling_books
Research project to synthesise the human genome
The Genome Project–Write (also known as GP-Write) is a large-scale collaborative research project (an extension of Genome Projects, aimed at reading genomes
Genome_Project–Write
Imperative, procedural programming language
callee) which is implemented very efficiently as sequences have automatic copy-on-write semantics. In other words, when you pass a sequence to a routine, initially
Euphoria (programming language)
Euphoria_(programming_language)
Write Ahead Physical Block Logging (WAPBL) provides meta data journaling for file systems in conjunction with Fast File System (FFS) to accomplish rapid
Write Ahead Physical Block Logging
Write_Ahead_Physical_Block_Logging
Finnish software company
interruptions. It uses a copy‑on‑write architecture with transaction points to ensure data consistency and controlled write behavior. NitroFS is intended
Tuxera
Software architecture design pattern
storageMap = new HashMap<>(); private final CopyOnWriteArrayList<Consumer<String>> observers = new CopyOnWriteArrayList<>(); public void setValue(String
Mediator_pattern
Software optimization technique
updates. Another example of laziness in modern computer systems is copy-on-write page allocation or demand paging, where memory is allocated only when
Lazy_evaluation
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