|This article relies largely or entirely upon a single source. (January 2013)|
Copy-on-write (sometimes referred to as "COW") is an optimization strategy used in computer programming. Copy-on-write stems from the understanding that when multiple separate tasks use identical copies of the same information (i.e., data stored in computer memory or disk storage), it is not necessary to create separate copies of that information for each process, instead they can all be given pointers to the same resource. When there are many separate processes all using the same resource it is possible to make significant resource savings by sharing resources this way. However, when a local copy has been modified, the copy-on-write paradigm has no provision that the shared resource has in the meantime not been updated by another task or tasks. Copy-on-write is therefore amenable if only the latest update is important and occasional use of a slightly stale value is not harmful. Copy-on-write is the name given to the process of identifying when a task attempts to make a change to the shared information, creating a separate (private) copy of that information for the task and redirecting the task to making changes to the private copy to prevent its changes from becoming visible to all the other tasks. All of this happens within the operating system kernel making the process transparent to both the task requesting the change and the other tasks using the shared copy.
Copy-on-write in virtual memory management
Copy-on-write finds its main use in virtual memory operating systems; when a process creates a copy of itself, the pages in memory that might be modified by either the process or its copy are marked copy-on-write. When one process modifies the memory, the operating system's kernel intercepts the operation and copies the memory thus a change in the memory of one process is not visible in another's.
Another use involves the calloc function. This can be implemented by means of having a page of physical memory filled with zeros. When the memory is allocated, all the pages returned refer to the page of zeros and are all marked copy-on-write. This way, the amount of physical memory allocated for the process does not increase until data is written. This is typically done only for larger allocations.
Copy-on-write can be implemented by notifying the MMU that certain pages in the process's address space are read-only. When data is written to these pages, the MMU raises an exception which is handled by the kernel, which allocates new space in physical memory and makes the page being written correspond to that new location in physical memory.
One major advantage of COW is the ability to use memory sparsely. Because the usage of physical memory only increases as data is stored in it, very efficient hash tables can be implemented which only use little more physical memory than is necessary to store the objects they contain. However, such programs run the risk of running out of virtual address space — virtual pages unused by the hash table cannot be used by other parts of the program. The main problem with COW at the kernel level is the complexity it adds, but the concerns are similar to those raised by more basic virtual-memory concerns such as swapping pages to disk; when the kernel writes to pages, it must copy any such pages marked copy-on-write.
COW may also be used as the underlying mechanism for disk storage snapshots such as those provided by logical volume management, Microsoft Volume Shadow Copy Service or file systems such as btrfs in Linux.
Other applications of copy-on-write
COW is also used outside the kernel, in library, application and system code. The string class provided by the C++ standard library, for example, was specifically designed to allow copy-on-write implementations in the C++98/03 standards, but not in the newer C++11 standard:
std::string x("Hello"); std::string y = x; // x and y use the same buffer y += ", World!"; // now y uses a different buffer // x still uses the same old buffer
In multithreaded systems, COW can be implemented without the use of traditional locking and instead use compare-and-swap to increment or decrement the internal reference counter. Since the original resource will never be altered, it can safely be copied by multiple threads (after the reference count was increased) without the need of performance-expensive locking such as mutexes. If the reference counter turns 0, then by definition only 1 thread is holding a reference so the resource can safely be de-allocated from memory, again without the use of performance-expensive locking mechanisms. The benefit of not having to copy the resource (and the resulting performance gain over traditional deep-copying) will therefore be valid in both single- and multithreaded systems.
Copy-on-write is also used in maintenance of instant snapshot on database servers like Microsoft SQL Server 2005. Instant snapshots preserve a static view of a database by storing a pre-modification copy of data when underlying data are updated. Instant snapshots are used for testing uses or moment-dependent reports and should not be used to replace backups. On the other hand, snapshots enable database back-ups in a consistent state without taking them offline.
- Kasampalis, Sakis (2010). "Copy On Write Based File Systems Performance Analysis And Implementation" (pdf). p. 19. Retrieved 11 January 2013.