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Download Peace of Mind: Making DBCC CHECKDB Go Faster
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Making DBCC CHECKDB Go Faster Argenis Fernandez Senior Database Engineer SurveyMonkey [email protected] @DBArgenis http://www.sqlblog.com/blogs/argenis_Fernandez/ About your Speaker Today… • Member of the Database Engineering team at SurveyMonkey • Formerly: Senior Consultant, Microsoft Consulting Services & Senior DBA at Outerwall and Ultimate Software • Microsoft Certified Master: SQL Server 2008, Microsoft Certified Solutions Master: Charter - Data Platform • 2014 Friend of Red Gate Software • 2012-2013 Idera SQL Server ACE • DBA/Dev/SysAdmin for 15 years • Loves Windows and SQL Server Internals • Regular Speaker (PASS Summit, PASS SQLRally, SQLBits XI, TechEd 2013, IT/DevConnections) • Twitter enthusiast; infrequent blogger. Why Am I Qualified To Speak About This? • My name is not Paul Randal • I took consistency checks on our 8.4TB database from 9+ days down to 10 hours on slower storage, 6 hours on an all-flash array • Note: This session lasts < 60 minutes, so we are not going to watch paint dry Making DBCC CHECKDB Go Faster • Get the biggest server you can find with the fastest CPUs, all the RAM you can pack on it and all PCI-Express based storage • Tune Windows • Tune SQL Server • You’re Done! Consistency Checks? Why Should I Care? What Causes Corruption? What Should Be My Goal? What Am I Trying To Achieve? What Should Be My Goal? • To try to make DBCC CHECKDB run as fast as possible? • No, you cannot really measure as fast as possible. • Use a baseline, but which one? • BACKUP DATABASE <db> TO DISK = ‘NUL’ • That is your theoretical “limit” What Kind Of Hardware Do I Need? What Kind Of Hardware Do I Need? • Obviously the fastest, biggest the better, but: • It’s perfectly okay to offload consistency checks to a smaller, slower box knowing that it’ll take longer, but you will not impact your production instance AS LONG as you run WITH PHYSICAL_ONLY checks on the primary/prod instance • In that case, it can be commodity hardware Offloading Consistency Checks Where Is It Okay To Offload Consistency Checks To? • Log Shipping Secondary on STANDBY? • Snapshot on a Mirroring Partner? • Replication Subscriber? • AlwaysOn Availability Groups Readable Secondary? • Restored copy of the prod database from a fresh FULL backup? • SAN Snapshot? What Does CHECKDB Actually Do? Breakdown of DBCC CHECKDB Tasks • DBCC CHECKALLOC • DBCC CHECKTABLE • DBCC CHECKCATALOG • Checks metadata and filesystem directories/files for FILESTREAM • Checks Service Broker structures • Breaking down these processes separately might be a good choice if you have extremely large databases Great, But How Do I Make It Go Faster?? Check Your Power Plan On Windows • Default is “Balanced” – awful • Set to “High Performance” • Use powercfg.cpl or powercfg.exe to correct • Consider disabling all power saving features in your server’s BIOS/UEFI settings • Verify you are okay by leveraging CPU-Z from http://www.cpuid.com tempdb Defaults Suck. Fix them. • tempdb’s default data file size sucks (8MB data/1MB log) • Its autogrowth settings suck (10% for both files) • The number is tempdb files is usually not optimal • • • • PFS/SGAM/GAM contention can be observed in a lot of systems Increase to # of files = # of cores to begin with for <= 8 cores For > 8 cores increase by 4 as long as you see contention For a good brain-melting discussion on tempdb, watch Bob Ward’s “Inside Tempdb” talk at the 2011 PASS Summit, available here: bit.ly/1gF3qi6 • Run DBCC CHECKDB WITH ESTIMATEONLY to get an idea of how much tempdb space consistency checks will require The Default MAXDOP Sucks. Fix It. • CHECKDB can and will go parallel on Enterprise Edition • MAXDOP default is zero • Change to number of cores per NUMA node • Use SysInternals’ COREINFO.EXE to find out how many Cores/NUMA node in your system • Check out Paul Randal’s blog post on CHECKDB DOP scalability • http://www.sqlskills.com/blogs/paul/dbcc-checkdb-scalability-andperformance-benchmarking-on-ssds/ • You can choose to turn off parallelism for CHECKDB with Trace Flag 2528 Consider BYOS: Bring Your Own Snapshot • Applies to Enterprise Edition only • CHECKDB creates an internal snapshot to obtain a consistent view of the database before it performs any checks • You can choose to roll your own snapshot and run CHECKDB against it • You can choose to keep the NTFS alternate stream (copy-on-write) files on different storage Consider Using WITH TABLOCK, go SingleUser, or Read-Only • Probably not feasible in your environment • Database is not available to regular users • No snapshot needed, nor one is created Consider Using Trace Flags • Trace Flags 2549 and 2562 were specifically created to increase performance of CHECKDB on very large databases • Documented in this KB: http://support.microsoft.com/kb/2634571 • Aaron Bertrand has a list of builds that support these trace flags here: http://www.sqlperformance.com/2012/11/io-subsystem/minimize-impact-ofcheckdb - the _entire_ article is a must read • Although developed specifically to increase WITH PHYSICAL_ONLY performance, the KB acknowledges that they can also increase the performance of full checks – and we have observed that they do help a bit in our environment • [CAUTION] Test carefully – these might have a negative impact on performance! Consider Disabling/Dropping Certain NonClustered Indexes • On Computed Columns • http://www.sqlskills.com/blogs/paul/dbcc-checkdb-performance-andcomputed-column-indexes/ • Filtered on Sparse Columns • http://sqlblog.com/blogs/argenis_fernandez/archive/2013/06/16/dbcccheckdb-on-vvldb-and-latches-or-my-pain-is-your-gain.aspx • Blame the expression evaluator Consider Limiting The Memory Available to CHECKDB • Wait, WHAT??? • CHECKDB grossly overestimates the amount of memory it will need to execute, stealing pages from the Buffer Pool and decreasing overall performance • Use Resource Governor to limit. Enterprise Edition only • http://www.sqlskills.com/blogs/jonathan/dbcc-checkdb-executionmemory-grants-not-quite-what-you-expect/ • Our tests point to a 1GB of execution memory is usually optimal Questions? Thank You! Resources • Ola Hallengren’s Maintenance solution: http://ola.hallengren.com • Paul Randal’s blog, CHECKDB category: http://www.sqlskills.com/blogs/paul/category/checkdb-from-everyangle/ Contact Information [email protected] @DBArgenis http://www.sqlblog.com/blogs/argenis_Fernandez/
