I think it is better to rephrase: Why does new distributed VoltDB use a command log over write-ahead log? Undoubtedly you are advanced enough to abstract a file system and use block storage along with some additional optimizations.
The transaction log is a critical component of the database and, if there is a system failure, the transaction log might be required to bring your database back to a consistent state. This guide provides information about the physical and logical architecture of the transaction log.
Understanding the architecture can improve your effectiveness in managing transaction logs. Each log record is identified by a log sequence number LSN. Each new log record is written to the logical end of the log with an LSN that is higher than the LSN of the record before it.
Log records are stored in a serial sequence as they are created. Each log record contains the ID of the transaction that it belongs to. For each transaction, all log records associated with the transaction are individually linked in a chain using backward pointers that speed the rollback of the transaction.
Log records for data modifications record either the logical operation performed or they record the before and after images of the modified data. The before image is a copy of the data before the operation is performed; the after image is a copy of the data after the operation has been performed.
The steps to recover an operation depend on the type of log record: Logical operation logged To roll the logical operation forward, the operation is performed again.
To roll the logical operation back, the reverse logical operation is performed. Before and after image logged To roll the operation forward, the after image is applied. To roll the operation back, the before image is applied.
Many types of operations are recorded in the transaction log. The start and end of each transaction. Every data modification insert, update, or delete. This includes changes by system stored procedures or data definition language DDL statements to any table, including system tables.
Every extent and page allocation or deallocation. Creating or dropping a table or index. Rollback operations are also logged. Each transaction reserves space on the transaction log to make sure that enough log space exists to support a rollback that is caused by either an explicit rollback statement or if an error is encountered.
The amount of space reserved depends on the operations performed in the transaction, but generally it is equal to the amount of space used to log each operation. This reserved space is freed when the transaction is completed. The section of the log file from the first log record that must be present for a successful database-wide rollback to the last-written log record is called the active part of the log, or the active log.
This is the section of the log required to a full recovery of the database. No part of the active log can ever be truncated.
Transaction Log Physical Architecture The transaction log in a database maps over one or more physical files. Conceptually, the log file is a string of log records. Physically, the sequence of log records is stored efficiently in the set of physical files that implement the transaction log.
There must be at least one log file for each database.Write-Ahead Logging The next optimization that I tried was something called “Write-Ahead Loggging” journaling mode (Also known as WAL mode). Journaling is a technique that SQLite uses to ensure that the database does not become corrupt in the event of a power failure or system crash.
Alphabetical List Of Documents See Also: Categorical Document List; Books About SQLite; Permuted Title Index; Website Keyword Index. 35% Faster Than The Filesystem; 8+3 Filenames; About SQLite; Alphabetical List Of SQLite Documents; An Asynchronous I/O Module For SQLite; Write-Ahead Logging;.
Write-Ahead log contains all changed data, Command log will require addition processing, but fast and lightweight. VoltDB: Command Logging and Recovery The key to command logging is that it logs the invocations, not the consequences, of the transactions. 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 system s.
In a system using WAL, all modifications are written to a log before they are applied. Nov 05, · High Level Write Ahead Logging. High Level Write Ahead Logging. Tuning PostgreSQL for High Write Workloads - Duration: Oracle Undo & Redo Explained - Duration: John Boyle 4, views.
Write-Ahead Transaction Log. This section describes the role of the write-ahead transaction log in recording data modifications to disk.
SQL Server uses a write-ahead log (WAL), which guarantees that no data modifications are written to disk before the associated log record is written to disk. This maintains the ACID properties for a transaction.