Uploaded byPARIKSHIT SAVJANI
Indirect checkpointing
The document discusses SQL Server 2012's new indirect checkpoint algorithm. The current checkpoint algorithm flushes all dirty buffers during a checkpoint, causing unpredictable recovery times and IO spikes. The indirect checkpoint algorithm calculates a new minimum recovery LSN during checkpoints instead of flushing buffers. A background recovery writer thread then flushes pages when the dirty page count exceeds a threshold, maintaining predictable recovery times without heavy IO during checkpoints. The document recommends identifying databases that need predictable recovery times and testing indirect checkpoints before production use.
Indirect checkpointing
- 1. SQL 2012: SQL Server Day Indirect Checkpoints Parikshit Savjani Premier Field Engineer, Explained Microsoft http://www.sqlserverfaq.net
- 2. Agenda Current Checkpoint Checkpoints Algorithm Indirect Checkpoint Challenges Algorithm Call to Action http://www.sqlserverfaq.net
- 3. Update A=C T Update B=D L commit O G Dirty Dirty B BUFFER Update POOL U RAM C D commit F F E R CHECKPOINT Dirty Dirty C B A D Insert Delete http://www.sqlserverfaq.net HARD DRIVE Userdb.mdf Userdb.ldf
- 4. CURRENT CHECKPOINT ALGORITHM Userdb.ldf Checkpoint (Flush Dirty Buffers) Delete 50 pages Recovery Interval = 1 min Update 10 pages Insert 100 pages Checkpoint (Flush Dirty Buffers) Update 10000 pages But Recovery is dependent on Number Of Dirty Buffers to be Insert 1000 pages CRAS flushed which is Not Accounted. H Delete 500 pages Checkpoint (Flush Dirty Buffers) http://www.sqlserverfaq.net
- 5. Checkpoint IO: ExistingAlgorithm http://www.sqlserverfaq.net
- 6. Challenges with theCurrent Checkpoint Algorithm Checkpoint Checkpoint Recovery Time Common to see process takes doesn’t is not IO spikes at the time searching account no. of predictable and Checkpoint for dirty buffers dirty buffers to cannot be Operation while scanning be flushed guaranteed the buffer pool http://www.sqlserverfaq.net
- 7. INDIRECT CHECKPOINT ALGORITHM LSN 0 Checkpoint MinLSN 50 Userdb.ldf LSN 10 pages Dirty Page List 10 LSN 20 pages LSN 0: Checkpoint LSN LSN 30 100 pages Checkpoint LSN 10: Delete 50 pages DIRTY_PAGE_POLL Target Recovery Time = 1 min LSN 20: Update 10 pages Background Recovery Writer LSN 30: Insert 100 pages If Dirty Pages > Threshold Checkpoint Then Flush Pages() LSN 40:Update 10000 pages Move MinLSN forward Target Dirty Buffer Threshold = 60 pages LSN 50: Insert 1000 pages NO Pages Are Flushed during LSN 60: Delete 500 pages Checkpoint Checkpoint Instead it just calculates the New MinRecoveryLSN http://www.sqlserverfaq.net
- 8. Goals of theIndirect Checkpoint Algorithm Recovery Time should be predictable and should be guaranteed Reduce or Smoothen the IO spikes caused by automatic checkpoint http://www.sqlserverfaq.net
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- 10. POINTS TO REMEMBER • Indirect Checkpoints are turned off by default • Indirect Checkpoints needs to enabled for each database explicitly using the new command syntax • ALTER DATABASE … SET TARGET_RECOVERY_TIME = target_recovery_time { SECONDS | MINUTES } • When Indirect Checkpoint is in effect, the checkpoint background thread • Does not perform the FlushCache work • just calculates MinRecoveryLSN and writes it to Tlog • A new background thread called RECOVERY WRITER (one per sql instance) is available to flush the pages when dirty pages reaches the threshold • A new internal waitype called DIRTY_PAGE_POLL is introduced as RECOVERY WRITER is timed thread to periodically check Dirty Page List http://www.sqlserverfaq.net
- 11. CALL TO ACTION • Identify the databases in your environment which requires predictable and guaranteed RTO • Test the IO Performance for the given database before turning it ON in production • Setting the target_recovery_time too small increases the IO workload and causes recovery writer thread to flush aggressively • Tune target_recovery_time to strike the right balance between IO workload and RTO • Use the new perfmon counter SQL Server:Buffer ManagerBackground Writer Pages/Sec and Extended Events for monitoring http://www.sqlserverfaq.net
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