Thursday 18 July 2013

SQL Server - Difference between Checkpoint and Lazy writer in SQL Server.

Please find the 18 differences between Checkpoint and Lazy Writer in SQL Server. 

SNo
CHECKPOINT
LAZY WRITER
1
Checkpoint is used by sql engine to keep database recovery time in check
Lazy writer is used by SQL engine only to make sure there is enough memory left in sql buffer pool to accommodate new pages
2
Check point always mark entry in T-log before it executes either sql engine or manually
Lazy writer doesn’t mark any entry in T-log
3
Checkpoint only check if page is dirty or not
Lazy writer clears any page from memory when it satisfies all of 3 conditions.
1. Memory is required by any object and available memory is full
2. Cost factor of page is zero
3. Page is not currently reference by any connection
4
Checkpoint is affected by two parameters
1. Checkpoint duration: is how long the checkpoint can run for.
2. Recovery interval: affects how often it runs.
Lazy writer is affected by
1. Memory pressure
2. Reference counter of page in memory
5
Flush dirty pages to Disk 
Flush dirty pages to disk. 
6
Flush only Data pages to disk 
Check for available memory and removed Buffer pool (execution plan/compile plan/ Data pages /Memory objects)
7
Default, Occurs approximately every 1 minute, run as per defined frequency
Occurs depending upon memory pressure and resource availability
8
Can be managed with sp_confige -recovery interval option
It is lazy; SQL Server manages by its own. 
9
Does not check the memory pressure 
Monitor the memory pressure and try maintaining the available free memory. 
10
Crash recovery process will be fast to read log as data file is updated.
No role in recovery 
11
Occurs for any DDL statement 
Occurs per requirement
12
Occurs before Backup/Detach command 
Occurs per requirement
13
Depends upon the configuration setting, we can control. 
Works on Least recent used pages and removed unused plans first, no user control. 
14
for simple recovery it flush the tlog file after 70% full. 
No effect on recovery model.
15
can manually /Forcefully run command “Checkpoint” 
No command for Lazy Writer 
16
Very Less performance impact 
No performance impact
17
Members of the SYSADMIN, DB_OWNER and DB_BACKUPOPERATOR can execute checkpoint manually
Not Applied
18
To get checkpoint entry in error log DBCC TRACEON(3502, -1)
Not Applied
19
Checkpoint is affected by Database recovery model
Lazy writer doesn’t get impacted with recovery model of database
20
Auto frequency can be controlled using recovery interval in sp_configure
Works only @ memory pressure , It uses clock algorithm for cleaning buffer cache
21
It keeps no. of dirty pages in memory to minimum
It helps to reduce paging


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