computer science Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...

, shadow paging is a technique for providing atomicity and durability (two of the

ACID An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

properties) in

database system In computing, a database is an organized collection of data or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, applications, and the database itself to capture and anal ...

s. A ''page'' in this context refers to a unit of physical storage (probably on a

hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk drive platter, pla ...

), typically of the order of 1 to 64 KiB. Shadow paging is a

copy-on-write Copy-on-write (COW), also called implicit sharing or shadowing, is a resource-management technique used in programming to manage shared data efficiently. Instead of copying data right away when multiple programs use it, the same data is shared ...

technique for avoiding

in-place In computer science, an in-place algorithm is an algorithm that operates directly on the input data structure without requiring extra space proportional to the input size. In other words, it modifies the input in place, without creating a separa ...

updates of pages. Instead, when a page is to be modified, a ''shadow page'' is allocated. Since the shadow page has no references (from other pages on disk), it can be modified liberally, without concern for consistency constraints, etc. When the page is ready to become durable, all pages that referred to the original are updated to refer to the new replacement page instead. Because the page is "activated" only when it is ready, it is atomic. If the referring pages must also be updated via shadow paging, this procedure may recurse many times, becoming quite costly. One solution, employed by the

Write Anywhere File Layout The Write Anywhere File Layout (WAFL) is a proprietary file system that supports large, high-performance Redundant array of independent disks, RAID arrays, quick restarts without lengthy consistency checks in the event of a Crash (computing), c ...

(WAFL) file system, is to be lazy about making pages durable (i.e., write-behind caching). This increases performance significantly by avoiding many writes on hotspots high up in the referential hierarchy (e.g., a file system superblock) at the cost of high commit latency.

Write-ahead logging In computer science, write-ahead logging (WAL) is a family of techniques for providing atomicity and durability (two of the ACID properties) in database systems. A write ahead log is an append-only auxiliary disk-resident structure used for cras ...

is a more popular solution that uses in-place updates. Shadow paging is similar to the old master–new master batch processing technique used in mainframe database systems. In these systems, the output of each batch run (possibly a day's work) was written to two separate disks or other form of storage medium. One was kept for backup, and the other was used as the starting point for the next day's work. Shadow paging is also similar to

purely functional data structure In computer science, a purely functional data structure is a data structure that can be directly implemented in a purely functional language. The main difference between an arbitrary data structure and a purely functional one is that the latter ...

s, in that in-place updates are avoided.

References

{{Reflist Database algorithms Computer file systems