Copy-on-write: Difference between revisions

Revision as of 17:42, 16 December 2024

Revision as of 15:02, 17 May 2025

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==In computer storage==

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

In traditional file systems, file changes overwrite the original data. With COW, when changes are made, a new version of the file is created while keeping the original intact. This approach enables features like [[Snapshot (computer storage)|snapshots]], which capture the state of a file at a specific time without consuming much additional space. Snapshots typically store only the modified data and are kept close to the original. However, they are considered a weak form of [[incremental backup]] and cannot replace 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>