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A modified page is written to a different location, and a new version of the parent pages in the tree is created, pointing at the new location.

From

Cover of Designing Data-Intensive Applications

Note

When implementing snapshots, there are two techniques: The original storage is never modified. When a write request is made, it is redirected away from the original data into a new storage area. (called "Redirect-on-write" or ROW) When a write request is made, the data are copied into a new storage area, and then the original data are modified. (called "Copy-on-write" or CoW) Despite their names, copy-on-write usually refers to the first technique. CoW does two data writes compared to ROW's one; it is difficult to implement efficiently and thus used infrequently.