Overview The default method by which SQLite implements atomic commit and rollback is a rollback journal.
What happens if in the middle of your update the server or your application crashes — what is your expected result when you revisit your data?
For more information on ACID see: What do we mean by Write Ahead Logging?
The page containing the existing data row is fetched into the Buffer Pool, a transaction start marker is written to the transaction log to indicate a transaction for this session has begun; the data is then modified in the Buffer Pool, the modified page is written to the transaction log and on commit a transaction end marker placed in the log.
At this point there has been no writes to the data file, the modified data is physically on storage in the transaction log file and in memory in the Buffer Pool.
What happens if SQL Server now fails i. When SQL Server starts up and the Database goes through the recovery process the transaction log is read sequentially bringing the data files up-to-date with any uncommitted transactions rolled back and any committed transactions rolled forward, the database is now in a consistent state.
The LSN is a key piece of information and critical to the consistency of data within the Database, you will even find a LSN in the page header records of data.
For more information see: There is a restriction: For example, we do a modification that changes pages 1 —remember WAL requires that those modifications be written to the transaction log, so we now have dirty pages for example 1 — in the Buffer Pool and for the purpose of this example operations on the transaction log for example LSN — See an animation of the overall concept of the Checkpoint process.
Eagerwriter For minimally logged operations for example BULK INSERT the eagerwriter writes dirty pages to storage without waiting for the operations to complete which might starve the available buffers. Data is written in the Buffer Pool then to the transaction log, the transaction log contains details of every operation that modifies database pages and thus anything in the databasethe transaction log can be used to recover the Data Files to a known state — rolling forward transactions or rolling them back.write to log file information “Will write this data (here goes the data) to this file (path) at offset (offset)" close the log file make sure that log file got actually written to disk.
Write a message to the server log if checkpoints caused by the filling of checkpoint segment files happen closer together than this many seconds (which suggests that checkpoint_segments ought to be raised).
The transaction log is used to apply the Atomicity (all or nothing) and Durability (when it’s written it’s definitely written) rules in ACID, the next section on Write Ahead Logging (WAL) explains how.
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 write-ahead log is sequence of append-only files containing all the write operations that were executed on the server. It is used to run data recovery after a server crash, and can also be used in a replication setup when slaves need to replay the same sequence of operations as on the master.
SQL Server Transaction Log – Part 1 – Log Structure and Write-Ahead Logging (WAL) Algorithm December 18, by Miroslav Dimitrov SQL Server transaction log is one of the most critical and in the same time one of the most misinterpreted part.