Copy-on-write: Difference between revisions

Revision as of 13:28, 15 September 2024

Revision as of 10:50, 6 October 2024

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{{Use dmy dates|date=November 2023}}

'''Copy-on-write''' ('''COW'''), sometimesalso referred to ascalled '''implicit sharing'''<ref>{{cite web |title=Implicit Sharing |url=https://doc.qt.io/qt-5/implicit-sharing.html |website=Qt Project |access-date=10 November 2023 |archive-date=8 February 2024 |archive-url=https://web.archive.org/web/20240208175543/https://doc.qt.io/qt-5/implicit-sharing.html |url-status=live }}</ref> or '''shadowing''',<ref>{{cite journal |last=Rodeh |first=Ohad |title=B-Trees, Shadowing, and Clones |journal=ACM Transactions on Storage |volume=3 |issue=4 |date=1 February 2008 |page=1 |citeseerx=10.1.1.161.6863 |s2cid=207166167 |doi=10.1145/1326542.1326544 |url=http://liw.fi/larch/ohad-btrees-shadowing-clones.pdf |archive-url=https://web.archive.org/web/20170102212904/http://liw.fi/larch/ohad-btrees-shadowing-clones.pdf |archive-date=2 January 2017 |access-date=10 November 2023 }}</ref>, is a [[Resource management (computing)|resource-management]] technique used in [[computer programming]] to efficiently implement a "duplicate" or "copy" operation on modifiable [[System resource#General resources|resources]]<ref name="Linux">{{cite book |last1=Bovet |first1=Daniel Pierre |last2=Cesati |first2=Marco |date=1 January 2002 |title=Understanding the Linux Kernel |url=https://books.google.com/books?id=9yIEji1UheIC&q=%22copy%20on%20write%22&pg=PA295 |publisher=O'Reilly Media |isbn=9780596002138 |page=295 |access-date=10 November 2023 |archive-date=15 September 2024 |archive-url=https://web.archive.org/web/20240915132745/https://books.google.com/books?id=9yIEji1UheIC&q=%22copy%20on%20write%22&pg=PA295#v=snippet&q=%22copy%20on%20write%22&f=false |url-status=live }}</ref> (mostused commonlyin memory[[computer pages,programming|programming]] storageto sectors,manage files, andshared data structures)efficiently.

Instead of copying data immediately when multiple programs use it, the data is shared until one program tries to change it. At that point, a copy is made, saving [[System resource#General resources|resources]]<ref name="Linux">{{cite book |last1=Bovet |first1=Daniel Pierre |last2=Cesati |first2=Marco |date=1 January 2002 |title=Understanding the Linux Kernel |url=https://books.google.com/books?id=9yIEji1UheIC&q=%22copy%20on%20write%22&pg=PA295 |publisher=O'Reilly Media |isbn=9780596002138 |page=295 |access-date=10 November 2023 |archive-date=15 September 2024 |archive-url=https://web.archive.org/web/20240915132745/https://books.google.com/books?id=9yIEji1UheIC&q=%22copy%20on%20write%22&pg=PA295#v=snippet&q=%22copy%20on%20write%22&f=false |url-status=live }}</ref> by avoiding unnecessary duplication until it's needed. This technique is commonly applied to memory, files, and data structures.

==In virtual memory management==

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==In computer storage==

COW is used as the underlying mechanism in file systems like [[ZFS]], [[Btrfs]], [[ReFS]], and [[Bcachefs]], as well as in [[logical volume management]] and database servers such as [[Microsoft SQL Server#Replication Services|Microsoft SQL Server]].

COW may also be used as the underlying mechanism for [[Snapshot (computer storage)|snapshots]], such as those provided by [[logical volume management]], file systems such as [[Btrfs]], [[ZFS]], [[ReFS]] and [[Bcachefs]],<ref>{{cite web |last=Kasampalis |first=Sakis |date=2010 |title=Copy-on-Write Based File Systems Performance Analysis and Implementation |page=19 |url=https://sakisk.me/files/copy-on-write-based-file-systems.pdf |access-date=10 November 2023 |archive-date=5 May 2024 |archive-url=https://web.archive.org/web/20240505220510/https://sakisk.me/files/copy-on-write-based-file-systems.pdf |url-status=live }}</ref> and database servers such as [[Microsoft SQL Server#Replication Services|Microsoft SQL Server]]. Typically, the snapshots store only the modified data, and are stored close to the original, so they are only a weak form of [[incremental backup]] and cannot substitute for a [[full backup]].<ref>{{cite web |last=Chien |first=Tim |title=Snapshots Are NOT Backups |url=https://www.oracle.com/database/technologies/rman-fra-snapshot.html |website=Oracle.com |publisher=Oracle |access-date=10 November 2023 |archive-date=10 November 2023 |archive-url=https://web.archive.org/web/20231110024434/https://www.oracle.com/database/technologies/rman-fra-snapshot.html |url-status=live }}</ref>

In traditional file systems, file changes overwrite the original data. With COW, maywhen alsochanges beare usedmade, asa new version of the underlyingfile mechanismis forcreated while keeping the original intact. This approach enables features like [[Snapshot (computer storage)|snapshots]], suchwhich ascapture thosethe providedstate byof [[logicala volumefile management]],at filea systemsspecific suchtime aswithout consuming much additional space. Snapshots typically store only the modified data and are kept close to the original. [[Btrfs]]However, [[ZFS]],they are considered a weak form of [[ReFSincremental backup]] and [[Bcachefs]],cannot replace a full backup.<ref>{{cite web |last=Kasampalis |first=Sakis |date=2010 |title=Copy-on-Write Based File Systems Performance Analysis and Implementation |page=19 |url=https://sakisk.me/files/copy-on-write-based-file-systems.pdf |access-date=10 November 2023 |archive-date=5 May 2024 |archive-url=https://web.archive.org/web/20240505220510/https://sakisk.me/files/copy-on-write-based-file-systems.pdf |url-status=live }}</ref> and database servers such as [[Microsoft SQL Server#Replication Services|Microsoft SQL Server]]. Typically, the snapshots store only the modified data, and are stored close to the original, so they are only a weak form of [[incremental backup]] and cannot substitute for a [[full backup]].<ref>{{cite web |last=Chien |first=Tim |title=Snapshots Are NOT Backups |url=https://www.oracle.com/database/technologies/rman-fra-snapshot.html |website=Oracle.com |publisher=Oracle |access-date=10 November 2023 |archive-date=10 November 2023 |archive-url=https://web.archive.org/web/20231110024434/https://www.oracle.com/database/technologies/rman-fra-snapshot.html |url-status=live }}</ref>

==See also==