Database engine performs the modifications to databases in memory also know on buffer cache and does not write these pages to disk after every change. To harden the changes to disk SQL Server issues periodic Checkpoints on each database. The checkpoint process is an automatic process which runs at specified intervals by SQL Server. Checkpoints will be issued by database. The Checkpoints will be given manually.
Why Checkpoint:
A checkpoint creates a known good point from which the SQL Server Database Engine can start applying changes contained in the log during recovery after an unexpected shutdown or crash.
When a checkpoint issued on a database no matter how it has occurred the same set of things will happen. Important things happen when a Checkpoint is issued:
1) The dirty pages (modified pages) will be hardened to physical Disk.
2) And it writes the log records from log buffer to physical file.
3) The LSN of the Checkpoint will be recorded in to the database boot page in the dbi_checkptLSN field.
The Database Engine supports four types of checkpoints:
1) Automatic Checkpoint: This will be Issued automatically in the background to meet the upper time limit suggested by the recovery interval server configuration option. The default recovery-interval value is 0, which allows the Database Engine to automatically configure the recovery interval. Typically, the default recovery interval results in automatic checkpoints occurring approximately once a minute for active databases and a recovery time of less than one minute. Higher values indicate the approximate maximum recovery time, in minutes. For example, setting the recovery interval to 3 indicates a maximum recovery time of approximately three minutes.
2) Indirect Checkpoint: This will be Issued in the background to meet a user-specified target recovery time for a given database. The default is 0, which indicates that the database will use automatic checkpoints, whose frequency depends on the recovery interval setting of the server instance. Setting the target recovery time to greater than 0 causes the database to use the indirect-checkpoints and establishes an upper-bound on recovery time for that particular database. We can find this option on database properties --> in Options tab --> in Recovery section.
3) Manual Checkpoints: Issued when you execute a Transact-SQL CHECKPOINT command. The manual checkpoint occurs in the current database for your connection. By default, manual checkpoints run to completion. Optionally, the checkpoint_duration parameter specifies a requested amount of time, in seconds, for the checkpoint to complete.
Command for manual Checkpoint:
CHECKPOINT [ checkpoint_duration ]
4) Internal checkpoints: These type of checkpoints issued by various server operations. Below is the list:
> Internal checkpoint will be issued during graceful shutdown of server.
> It will be issued when teh database recovery model changed from Full\Bulk-logged to Simple.
> it will issued during the database backup.
> It will be issued on DBs with simple recovery model when the T-log is 70% full.
> Alter Database command to add or remove a data\log file also initiates a checkpoint
> Checkpoint also takes place when the recovery model of the DB is Bulk-Logged and a minimally logged
operation is performed.
> It will be performed when a DB Snapshot created.
Factors Affecting the Duration of Checkpoint Operations:
In general, the amount time required for a checkpoint operation increases with the number of dirty pages that the operation must write. By default, to minimize the performance impact on other applications, SQL Server adjusts the frequency of writes that a checkpoint operation performs. Decreasing the write frequency increases the time the checkpoint operation requires to complete. SQL Server uses this strategy for a manual checkpoint unless a checkpoint_duration value is specified in the CHECKPOINT command.
The performance impact of using checkpoint_duration depends on the number of dirty pages, the activity on the system, and the actual duration specified. For example, if the checkpoint would normally complete in 120 seconds, specifying a checkpoint_duration of 45 seconds causes SQL Server to devote more resources to the checkpoint than would be assigned by default. In contrast, specifying a checkpoint_duration of 180 seconds would cause SQL Server to assign fewer resources than would be assigned by default. In general, a short checkpoint_duration will increase the resources devoted to the checkpoint, while a long checkpoint_duration will reduce the resources devoted to the checkpoint. SQL Server always completes a checkpoint if possible, and the CHECKPOINT statement returns immediately when a checkpoint completes. Therefore, in some cases, a checkpoint may complete sooner than the specified duration or may run longer than the specified duration.
I have gone through Paul Randal's article at www.sqlskills.com and two other sources as below.
Here are the links to the source articles for more details:
http://www.sqlskills.com/blogs/paul/how-do-checkpoints-work-and-what-gets-logged/
https://msdn.microsoft.com/en-us/library/ms189573.aspx
http://www.sarabpreet.com/2013/08/checkpoint/746
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