写入时复制（Copy-on-write，COW）是一种计算机编程领域的优化策略。其核心思想是，如果有多个调用者（callers）同时要求相同资源（如内存或磁盘上的数据存储），他们会共同取得相同的指标指向相同的资源，直到某个调用者试图修改资源的内容时，系统才会真正复制一份专用副本（private copy）给该调用者，而其他调用者所见到的最初的资源仍然保持不变。这过程对其他的调用者都是透明的（transparently）。此作法主要的优点是如果调用者没有修改该资源，就不会有副本（private copy）被创建，因此多个调用者只是读取操作时可以共享同一份资源。

Copy-on-write (sometimes referred to as "COW"), sometimes referred to as implicit sharing, is an optimization strategy used in computer programming. Copy-on-write stems from the understanding that when multiple separate tasks use initially identical copies of some information (i.e., data stored in computer memory or disk storage), treating it as local data, each task working on its own "copy of the data", that they may occasionally need to modify, then it is not necessary to immediately create separate copies of that information for each task. Instead they can all be given pointers to the same resource, with the provision that on the first occasion where they need to modify the data, they must first create a local copy on which to perform the modification (the original resource remains unchanged). When there are many separate processes all using the same resource, each with a small likelihood of having to modify it at all, then it is possible to make significant resource savings by sharing resources this way. Copy-on-write is the name given to the policy that whenever a task attempts to make a change to the shared information, it should first create a separate (private) copy of that information to prevent its changes from becoming visible to all the other tasks. If this policy be enforced by the operating system kernel, then the fact of being given a reference to shared information rather than a private copy can be transparent to all tasks, whether they need to modify the information or not.