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Transcript
Designing for Performance General Database Optimizations Presentation Highlight: The Anatomy of a Data Modification Wednesday, 12 November 2003 – 16:30-18:00 S207 Kimberly L. Tripp President, SYSolutions, Inc. – SQLSkills.com Email: [email protected] Principal Mentor, Solid Quality Learning – SolidQualityLearning.com Email: [email protected] Speaker – Kimberly L. Tripp Independent Consultant/Trainer/Speaker/Writer President, SYSolutions, Inc. www.SQLSkills.com Principal Mentor, Solid Quality Learning * In-depth, high quality training around the world! www.SolidQualityLearning.com SQL Server MVP (http://mvp.support.microsoft.com/) Microsoft Regional Director (http://www.microsoftregionaldirectors.com/Public/) Writer/Editor for TSQL Solutions/SQL Magazine www.tsqlsolutions.com and www.sqlmag.com Coauthor for MSPress title: SQL Server 2000 High Availability Presenter/Technical Manager for SQL Server 2000 High Availability Overview DVD (MS Part# 098-96661) General Database Performance 12 November 2003 16:30-18:00 The Anatomy of a Data Modification 1. User sends UPDATE 2. Update is highly selective (only 5 rows) Indexes exist to aid in finding these rows efficiently The update is a SINGLE statement batch NOT enclosed in BEGIN TRAN…COMMIT TRAN block therefore this is IMPLICIT transaction Server receives the request and locates the data in cache OR reads the data from disk into cache Since this is highly selective only the necessary pages are read into cache (maybe a few extra but that’s not important here) Let’s use an example where the 5 rows being modified are located on 3 different data pages General Database Performance 12 November 2003 16:30-18:00 What it looks like - Data Data Log UPDATE… Server… Cache General Database Performance 12 November 2003 16:30-18:00 The Anatomy of a Data Modification 3. SQL Server proceeds to lock the necessary data Locks are necessary to give us a consistent point FOR ALL rows from which to start If any other transaction(s) have ANY of these rows locked we will wait until ALL locks have been acquired before we can proceed. In the case of this update (because it’s highly selective and because indexes exist to make this possible) SQL Server will use row level locking. The rows are locked but there are also “intent” locks at higher levels to make sure other larger locks (like page or table level locks) are not attempted (and fail) There are a few locks that have already occurred – within indexes, etc. to read the data – but they are not significant here This sounds complex but it’s not too bad… General Database Performance 12 November 2003 16:30-18:00 What it looks like - Locks Update Lock Row Update Lock Page Row Update Lock Row Page Update Lock Cache Row Update Lock Page General Database Performance 12 November 2003 16:30-18:00 Row The Anatomy of a Data Modification 4. SQL Server can now begin to make the modifications – for EVERY row the process will include: 1. Change the lock to a stricter lock (eXclusive lock) 2. 3. An update lock helps to allow better concurrency by being compatible with other shared locks (readers). Readers can read the pre-modified data as it is transactionally consistent The eXclusive lock is required to make the change because once modified no other reads should be able to see this uncommitted change Make the modification to the data row (yes, in cache) Log the modification to the transaction log pages (also in cache) General Database Performance 12 November 2003 16:30-18:00 What it looks like - Modifications x Exclusive Update Lock Lock x Row x L Page x Row x x Exclusive Update Lock Lock Exclusive Update Lock Lock Row Page x Cache x x Row x Page General Database Performance 12 November 2003 16:30-18:00 Exclusive Update Lock Lock Row Exclusive Update Lock Lock The Anatomy of a Data Modification 5. Finally, the transaction is complete – this is the MOST critical step All rows have been modified There are no other statements in this transaction – i.e. Implicit transaction Steps are: 1. Write all log pages to transaction log ON DISK 2. Release the locks 3. Send a message to the user: (5 Rows Affected) General Database Performance 12 November 2003 16:30-18:00 What it looks like Write-Ahead Logging Data L Log 5 Rows Affected Server… Log After the log entries are made and the locks are released… General Database Performance 12 November 2003 16:30-18:00 ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~ Cache Sequential writes Change Change Change Change … So now what? The transaction log ON DISK – is up to date The data in CACHE – is up to date But when does the data get written from cache to disk? CHECKPOINT It’s important to realize that the sole purpose of checkpoint is NOT just to write committed pages… Instead a checkpoint writes ALL pages which have changed since they were brought into cache – regardless of the state of the transaction which changed them! General Database Performance 12 November 2003 16:30-18:00 Transaction Recovery and Checkpoints Transactions… 1 L 2 Action Required if restart recovery None D L/D L/D Roll forward L Roll back 3 4 L 5 Checkpoint Time General Database Performance 12 November 2003 16:30-18:00 Roll forward Roll back System Failure Resources Check out www.sqlskills.com for information about upcoming events, useful downloads and excellent scripts! There are quite a few resources and/or links to use. MSPress title: SQL Server 2000 High Availability Authors: Allan Hirt with Cathan Cook, Kimberly L. Tripp, Frank McBath ISBN: 0-7356-1920-4 Check out the main page of www.sqlskills.com for a sample chapter to download! General Database Performance 12 November 2003 16:30-18:00 Resources From Books Online “Home Page” select White Papers to get to msdn For Tech Net articles use: http://www.microsoft.com/technet/prodtec hnol/sql/default.asp?frame=true See www.microsoft.com/sql for all sorts of useful links, resources and whitepapers, etc. Support Resources listed: http://www.microsoft.com/sql/support/defa ult.asp General Database Performance 12 November 2003 16:30-18:00 Resources Whitepaper: Using Partitions in a Microsoft SQL Server 2000 Data Warehouse, http://msdn.microsoft.com/library/techart/partiti onsindw.htm Whitepaper: Index Tuning Wizard for Microsoft SQL Server 2000, http://msdn.microsoft.com/library/enus/dnsql2k/html/itwforsql.asp?frame=true Support WebCast: SQL Server 2000 Profiler: What's New and How to Effectively Use It http://support.microsoft.com/default.aspx?scid =%2Fservicedesks%2Fwebcasts%2Fwc111400 %2Fwcblurb111400%2Easp General Database Performance 12 November 2003 16:30-18:00 Resources Improving Performance with SQL Server 2000 Indexed Views http://msdn.microsoft.com/library/enus/dnsql2k/html/indexedviews1.asp?frame=tr ue Microsoft SQL Server 2000 Index Defragmentation Best Practices http://www.microsoft.com/technet/prodtechn ol/sql/maintain/Optimize/SS2KIDBP.asp?fram e=true Support Resources listed: http://www.microsoft.com/sql/support/default. asp General Database Performance 12 November 2003 16:30-18:00 Thank you! Kimberly L. Tripp President, SYSolutions, Inc. Website: www.SQLSkills.com Email: [email protected] Principal Mentor, Solid Quality Learning Website: www.SolidQualityLearning.com Email: [email protected] Please fill out your evaluation! General Database Performance 12 November 2003 16:30-18:00