Chapter Overview Understanding the Database Architecture Understanding the Transaction Log Architecture Understanding and Querying System and Database Catalogs

Data Files One primary and optional secondary data files for each database. Each data file is a separate operating system file. Primary data file contains all system tables for a database. Database file locations are stored in the master database and in the primary data file for each database. Each file has properties, such as file ID, initial file size, growth increment (if any), and maximum file size.

Space Allocation Data file space is allocated in extents and pages. Extents are either uniform or mixed. SGAM pages are used to locate allocated mixed extents with free space. GAM pages are used to locate unallocated extents for allocation to objects. IAM pages are used to locate allocated pages belonging to a table or index. PFS pages are used to locate allocated pages for an object with free space.

Index and Data Storage Data is stored in a heap unless a clustered index organizes table data. Indexes store key values from a table in a B-tree structure. If a clustered index is present, inserts can require data page reorganization.

Structure of a Clustered Index

Structure of a Nonclustered Index

Transaction Log Files Each database has one or more transaction log files. Each transaction log file is a separate operating system file. Transaction log files contain sequentially numbered log records containing database transactions. Each transaction log file and each growth increment contain two or more VLFs. Avoid too many VLFs by avoiding frequent small growth increments.

How the Transaction Log Works Data modifications are written to the transaction log before the data file. Transaction log records contain sufficient information to roll database modifications backward or forward if necessary. Dirty pages are written to disk by worker threads, a lazywriter thread, or the checkpoint process.

Checkpoint Process Ensures dirty pages in the memory buffer are periodically written to disk Writes all transactions older than MinLSN to disk Allows space in the transaction log to be reused Either truncates the transaction log or allows a transaction log backup to truncate the log

Log Truncation and Log Wrapping

Operating System Threads and Dirty Pages Ensure dirty pages in the buffer cache are periodically written to disk. Free memory buffer pages for reuse. Worker threads perform these tasks while waiting on their primary task. Lazywriter thread performs these tasks when free memory buffers run low. These two processes and the checkpoint process spread out disk writes and avoid disk bottlenecks.

Recovery Models Full Recovery model Bulk-Logged Recovery model Simple Recovery model

System Tables

System Table Query Methods Directly using Transact-SQL Indirectly using system stored procedures Indirectly using system functions Indirectly using information schema views

Chapter Summary Microsoft SQL Server databases consist of data files and transaction log files. Data files are organized using pages and extents, and specialty pages track allocated pages and extents. Clustered indexes order data; otherwise, data is unordered. Transaction logs ensure data consistency and provide levels of recoverability based on the recovery model. System tables record system and database object information.