A write-ahead log ensures that no data modifications are written to disk before the associated log record. SQL Server maintains a buffer cache into which it reads data pages when data must be retrieved. Data modifications are not made directly to disk, but are instead made to the copy of the page in the buffer cache. The modification is not written to disk until either the database is checkpointed, or the modifications must be written to disk so the buffer can be used to hold a new page.
Overview The default method by which SQLite implements atomic commit and rollback is a rollback journal. Beginning with version 3. There are advantages and disadvantages to using WAL instead of a rollback journal. WAL is significantly faster in most scenarios. WAL provides more concurrency as readers do not block writers and a writer does not block readers.
Reading and writing can proceed concurrently. WAL uses many fewer fsync operations and is thus less vulnerable to problems on systems where the fsync system call is broken.
But there are also disadvantages: All processes using a database must be on the same host computer; WAL does not work over a network filesystem. Transactions that involve changes against multiple ATTACHed databases are atomic for each individual database, but are not atomic across all databases as a set.
You must be in a rollback journal mode to change the page size. It is not possible to open read-only WAL databases. The opening process must have write privileges for "-shm" wal-index shared memory file associated with the database, if that file exists, or else write access on the directory containing the database file if the "-shm" file does not exist.
There is an additional quasi-persistent "-wal" file and "-shm" shared memory file associated with each database, which can make SQLite less appealing for use as an application file-format. There is the extra operation of checkpointing which, though automatic by default, is still something that application developers need to be mindful of.
WAL works best with smaller transactions. WAL does not work well for very large transactions. For transactions larger than about megabytes, traditional rollback journal modes will likely be faster. It is recommended that one of the rollback journal modes be used for transactions larger than a few dozen megabytes.
How WAL Works The traditional rollback journal works by writing a copy of the original unchanged database content into a separate rollback journal file and then writing changes directly into the database file.
In the event of a crash or ROLLBACKthe original content contained in the rollback journal is played back into the database file to revert the database file to its original state. The WAL approach inverts this.
The original content is preserved in the database file and the changes are appended into a separate WAL file. Thus a COMMIT can happen without ever writing to the original database, which allows readers to continue operating from the original unaltered database while changes are simultaneously being committed into the WAL.
Multiple transactions can be appended to the end of a single WAL file. Checkpointing Of course, one wants to eventually transfer all the transactions that are appended in the WAL file back into the original database.
Moving the WAL file transactions back into the database is called a "checkpoint". Another way to think about the difference between rollback and write-ahead log is that in the rollback-journal approach, there are two primitive operations, reading and writing, whereas with a write-ahead log there are now three primitive operations: Applications using WAL do not have to do anything in order to for these checkpoints to occur.
But if they want to, applications can adjust the automatic checkpoint threshold.
Or they can turn off the automatic checkpoints and run checkpoints during idle moments or in a separate thread or process. Concurrency When a read operation begins on a WAL-mode database, it first remembers the location of the last valid commit record in the WAL.
Call this point the "end mark". Because the WAL can be growing and adding new commit records while various readers connect to the database, each reader can potentially have its own end mark. But for any particular reader, the end mark is unchanged for the duration of the transaction, thus ensuring that a single read transaction only sees the database content as it existed at a single point in time.
The wal-index greatly improves the performance of readers, but the use of shared memory means that all readers must exist on the same machine.Write-Ahead Logging , whereas with a write-ahead log there are now three primitive operations: reading, writing, and checkpointing.
By default, SQLite does a checkpoint automatically when the WAL file reaches a threshold size of pages. "Write" and "log" are appropriate and factual--this is a log of my writing. Taken together, write-ahead-logging is used in many database systems, hinting this will be a software and programming centric blog.
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. HBase Architecture Write-Ahead Log.
What is the write-ahead log (WAL), you ask? In a previous article we looked at the general storage architecture of HBase.
One thing that was mentioned was the WAL. This post explains how the log works in detail, but bear in mind that it describes the current version, which is A write-ahead log ensures that no data modifications are written to disk before the associated log record.
SQL Server maintains a buffer cache into which it reads data pages when data must be. The concept of Write Ahead Logging is very common to database systems.
This process ensures that no modifications to a database page will be flushed to disk until the associated transaction log records with that modification are written to disk first.