Copy-on-write facts for kids

Copy-on-write (often called COW) is a clever way computers manage information. Imagine you have a favorite book, and your friend wants to read it too. Instead of buying a second copy right away, you both share the same book. If your friend just reads it, that's fine. But if your friend wants to write notes in it, they would then make their own copy of that page (or the whole book) to write in, so your original book stays clean.

That's how Copy-on-write works with computer data. When different parts of a computer program need to use the same information, the computer doesn't make copies immediately. Instead, they all share the original data. A copy is only made if one of the programs tries to change that data. This saves a lot of computer memory and makes programs run faster because they don't waste time copying things that might never be changed.

COW is used in many places, like managing computer memory, handling files, and organizing data.

How Copy-on-Write Works in Memory

One of the most important uses of Copy-on-write is in operating systems. These are the main programs that run your computer, like Windows or macOS. When you open a new program, the operating system often uses COW to share memory.

Sharing Memory for Programs

Think about when you open a new application. The computer creates a new "process" for it. Sometimes, this new process is very similar to an existing one. For example, when a program creates a copy of itself using a special command called `fork()`, the new copy often doesn't change its memory right away. It might just start a completely different task.

If the computer copied all the memory from the old program to the new one every time, it would be very slow and use too much memory. With Copy-on-write, the new process shares the old process's memory. If the new process doesn't change anything, no extra memory is used. If it does change something, only then is a copy of that specific part of the memory made for the new process. This makes starting new programs much quicker and more efficient.

How the Computer Manages COW Memory

The operating system uses something called a "page table" to manage memory. With COW, certain parts of memory are marked as "read-only." This means programs can look at the data but can't change it directly. The system also keeps track of how many programs are sharing each part of memory.

If a program tries to write to a read-only part of memory, the operating system steps in. It then creates a new, private copy of that memory section for the program that wants to make changes. The original memory section remains unchanged for other programs still sharing it. This way, each program gets its own version of the data when it needs to modify it, without affecting others.

COW can also help with giving programs memory that is initially empty. The system can have one special memory page filled with zeros. When a program asks for new memory, it's given access to this zero-filled page, marked as copy-on-write. Physical memory is only actually given to the program when it starts writing its own data into that space. This allows programs to ask for a lot of memory without actually using it all up until they really need it.

Copy-on-Write in Software Programs

COW isn't just for operating systems; it's also used in many other computer programs and software libraries.

Examples in Programming Languages

• C++ Strings: In older versions of the C++ programming language (like C++98), a special type of text called a `std::string` could use Copy-on-write. If you made a copy of a string, both copies would share the same text data. Only when one of them was changed would a separate copy be made. Newer versions of C++ (like C++11) don't allow this for strings anymore, but it's a good example of how it worked.
• PHP: In the PHP programming language, most types of data use Copy-on-write. For example, if you have a list of items (an array) and you pass it to a function, the function gets a reference to the original list. If the function changes the list, then a copy is made. This makes PHP programs run faster because they don't always copy large amounts of data.
• Qt Framework: The Qt framework, which is used to build many applications, uses COW for many of its data types. Qt calls this "implicitly shared." This means that when you copy a Qt object, it's a very fast operation because only a pointer to the data is copied, not the data itself. This is especially useful in programs that do many things at once (multithreading), as it can reduce the need for complex locking mechanisms.

Copy-on-Write in Computer Storage

COW is also a key part of how some modern file systems and storage systems work. File systems are how your computer organizes and stores files on your hard drive.

Modern File Systems

Traditional file systems often overwrite old data when you save changes to a file. With COW, when you make changes to a file, the system doesn't overwrite the original data. Instead, it writes the new changes to a different location and updates the file's record to point to the new data. The old data is kept safe.

This approach allows for cool features like "snapshots." A snapshot is like taking a picture of your file system at a specific moment in time. Because of COW, these snapshots don't take up much extra space. They only store the parts of the data that have changed since the snapshot was taken. This can be very useful for going back to an older version of a file if you make a mistake.

Some file systems that use COW include ZFS, Btrfs, and ReFS. It's also used in some database systems.

See also

In Spanish: Copy-on-write para niños

• Memory management
• Persistent data structure