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Definition: copy-on-write

A file system feature that makes modifications to separate copies of data when updating is required. Data being read are shared by pointers to one master set. However, when changes are made by one of the applications or users, a copy of the data is made, and the copy is updated. Eventually, the copy is written to the master set. Copy-on-write (COW) is used when data can be slightly out-of-date for a short period of time. See file system and Btrfs.