SQL Server Backup and Restore - Simple Talk

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Chapter 1: Basics of Backup and Restore for databases that are simply too large to back up in a single operation. However, they are not required on every database you manage. Unless you have experience with them, or know definitively that you will gain significant performance with their use, then sticking to a single data file database will work for you most of the time (with the data being automatically striped across physical storage, via RAID). Finally, before we move on, it's important to note that SQL Server transaction log files are never members of a filegroup. Log files are always managed separately from the SQL Server data files. Transaction log A transaction log file contains a historical account of all the actions that have been performed on your database. All databases have a transaction log file, which is created automatically, along with the data files, on creation of the database and is conventionally denoted with the LDF extension. It is possible to have multiple log files per database but only one is required. Unlike data files, where writes occur in a random fashion, SQL Server always writes to the transaction log file sequentially, never in parallel. This means that it will only ever write to one log file at a time, and having more than one file will not boost write-throughput or speed. In fact, having more multiple files could result in performance degradation, if each file is not correctly sized or differs in size and growth settings from the others. Some inexperienced DBAs don't fully appreciate the importance of the transaction log file, both to their backup and recovery plan and to the general day-to-day operation of SQL Server, so it's worth taking a little time out to understand how SQL Server uses the transaction log (and it's a topic we'll revisit in more detail in Chapter 5, Log Backups). Whenever a modification is made to a database object (via Data Definition Language, DDL), or the data it contains (Data Manipulation Language, DML), the details of the change are recorded as a log record in the transaction log. Each log record contains 24
Chapter 1: Basics of Backup and Restore details of a specific action within the database (for example, starting a transaction, or inserting a row, or modifying a row, and so on). Every log record will record the identity of the transaction that performed the change, which pages were changed, and the data changes that were made. Certain log records will record additional information. For example, the log record recording the start of a new transaction (the LOP_BEGIN_XACT log record) will contain the time the transaction started, and the LOP_COMMIT_XACT (or LOP_ABORT_XACT) log records will record the time the transaction was committed (or aborted). From the point of view of SQL Server and the DBA looking after it, the transaction log performs the following critical functions: • ensures transactional durability and consistency • enables, via log backups, point-in-time restore of databases. Transactional durability and consistency Via a Write Ahead Logging (WAL) mechanism that ensures that change descriptions are written to the transaction log before the actual data changes are written to the data files, SQL Server guarantees that all valid, committed data will be written to the data files and that the effects of any partial, uncommitted transactions, in the data file, can be "undone," via transaction rollback. As noted previously, the database's log file provides a record of all transactions performed against that database. When a data modification is made, the relevant data pages are read from the data cache, first being retrieved from disk if they are not already in the cache. Data is modified in the data cache, and the log records describing the effects of the transaction are created in the log cache. Any page in the cache that has been modified since being read from disk is called a "dirty" page. When a periodic CHECKPOINT operation occurs, all dirty pages, regardless of whether they relate to committed or uncommitted transactions, are flushed to disk. The WAL protocol dictates that, before a data page is 25
Page 1 and 2: The Red Gate Guide SQL Server Backu
Page 3 and 4: Copyright April 2012 ISBN 978-1-906
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Page 11 and 12: About the author Shawn McGehee Shaw
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USE [master] GO Chapter 9: File and
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Appendix A: SQL Backup Pro Installa
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SQL Server and .NET Tools from Red
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SQL Prompt Pro Write, edit, and exp
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SQL Monitor SQL Server performance
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SQL Toolbelt The essential SQL Serv
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.NET Reflector ® Decompile, browse
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Brad's Sure Guide to SQL Server Mai
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