Some [[live CD]]s (and [[live USB]]s) use copy-on-write techniques to give the impression of being able to add and delete files in any directory, without actually making any changes to the CD (or USB flash drive).
==In high-reliability software==
[[Phantom OS]] uses COW at all levels, not just a database or file system. At any time, a computer running this system can fail, and then, when it starts again, the software and operating system resume operation. Only small amounts of work can be lost.
The basic approach is that all program data are kept in virtual memory. On some schedule, a summary of all software data is written to virtual memory, forming a log that tracks the current value and ___location of each value.
When the computer fails, a recent copy of the log and other data remain safe on disk. When operation resumes, operating system software reads the log to restore consistent copies of all the programs and data.
This approach uses copy-on-write at all levels in all software, including in application software. This requires support within the application programming language. In practice, Phantom OS permits only languages that generate [[Java bytecode]].
