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.
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.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.
In a system using WAL, all modifications are written to a log before they are applied. 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. 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.
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.
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: reading, writing, and checkpointing.
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.