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A write ahead log is an append-only auxiliary disk-resident structure used for crash and transaction recovery. The changes are first recorded in the log, which must be written to stable storage, before the changes are written to the database. [2] The main functionality of a write-ahead log can be summarized as: [3]
Write-ahead logging: Any change to an object is first recorded in the log, and the log must be written to stable storage before changes to the object are written to disk. Repeating history during Redo: On restart after a crash, ARIES retraces the actions of a database before the crash and brings the system back to the exact state that it was in ...
In particular, the logging mechanism is called write-ahead log (WAL) and allows durability by buffering changes to the disk before they are synchronized from the main memory. In this way, by reconstruction from the log file, all committed transactions are resilient to system-level failures, because they can be redone.
Physically, a log is a file listing changes to the database, stored in a stable storage format. If, after a start, the database is found in an inconsistent state or not been shut down properly, the database management system reviews the database logs for uncommitted transactions and rolls back the changes made by these transactions ...
A database is both a physical and logical grouping of data. An ESE database looks like a single file to Windows. Internally the database is a collection of 2, 4, 8, 16, or 32 KB pages (16 and 32 KB page options are only available in Windows 7 and Exchange 2010), [1] arranged in a balanced B-tree structure. [2]
there is stable storage at each node with a write-ahead log, no node crashes forever, the data in the write-ahead log is never lost or corrupted in a crash, and; any two nodes can communicate with each other. The last assumption is not too restrictive, as network communication can typically be rerouted.
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Full copy-on-write file systems (such as ZFS and Btrfs) avoid in-place changes to file data by writing out the data in newly allocated blocks, followed by updated metadata that would point to the new data and disown the old, followed by metadata pointing to that, and so on up to the superblock, or the root of the file system hierarchy. This has ...