Database file configuration
When it comes to individual databases, there are a number of recommended configuration steps that SQL Server doesn’t perform, in large part due to dependencies on disk configuration and unknown future usage of the databases.
Before covering specific file configuration recommendations, let’s address some of the terms used when discussing database files:
* Primary data file —The primary data file, and by default the only data file, contains system tables and information on all files within a database. By default, this file has an .mdf extension. If there are no other files in the database, the primary file also contains user objects such as tables and indexes.
* Secondary data file —Secondary files, which usually have an .ndf extension, are optional files that can be added to a database for performance and/or administrative benefits, both of which we’ll cover shortly. A database can contain one or more secondary files.
* Filegroups —Every database contains a primary filegroup, containing at least the primary data file, and possibly all secondary data files unless other filegroups are created and used. Filegroups are logical containers that group together one or more data files, and as we’ll see later in the chapter, provide several benefits.
* Transaction log file —Typically using the .ldf extension, the transaction log file records details of each database modification and is used for various purposes, including transaction log shipping, replication, database mirroring, and recovery of a database to a consistent state. With these terms in mind, let’s cover some of the major file configuration recommendations, starting with separating a database’s different storage objects across separate physical disk volumes.
*** Volume separation
By default, a database is created with a single data and transaction log file. Unless specified during installation or modified during database creation, both of these files will be created in the same directory, with the default size and growth rates inherited from the model database.
As shown below, an important database file configuration task, particularly for databases with direct-attached storage, is to provide separate physical RAID-protected disk volumes for data, transaction log, tempdb, and backup files.
As designing SAN-based virtualized storage is quite different from designing direct-attached storage; that being said, the principles of high performance and fault tolerance remain. In both cases, a good understanding of SQL Server’s various storage objects is crucial in designing an appropriate storage system.
TRANSACTION LOG FILE:
Unlike random access to data files, transaction logs are written sequentially. If a disk is dedicated to a single database’s transaction log, the disk heads can stay in position writing sequentially, thus increasing transaction throughput. In contrast, a disk that stores a combination of data and transaction logs won’t achieve the same levels of throughput given that the disk heads will be moving between the conflicting requirements of random data access/updates and sequential transaction log entries. For database applications with high transaction rates, separation of data and transaction logs in this manner is crucial.
A common (and recommended) backup technique, covered in detail in the next chapter, is to back up databases to disk files and archive the disk backup files to tape at a later point in the day. The most optimal method for doing this is to have dedicated disk(s) for the purpose of storing backups.
Dedicated backup disks provide several benefits:
** Disk protection —Consider a case where the database files and the backup files are on the same disk. Should the disk fail, both the database and the backups are lost, a disastrous situation! Storing backups on separate disk(s) prevents this situation from occurring—either the database or the backups will be available.
** Increased throughput —Substantial performance gains come from multiple disks working in unison. During backup, the disks storing the database data files are dedicated to reading the files, and the backup disks are dedicated to writing backup file(s). In contrast, having both the data and backup files on the same disk will substantially slow the backup process.
** Cost-effective —The backup disks may be lower-cost, higher-capacity SATA disks, with the data disks being more expensive, RAID-protected SCSI or SAS disks.
** Containing growth —The last thing you want is a situation where a backup consumes all the space on the data disk, effectively stopping the database from being used. Having dedicated backup disks prevents this problem from occurring.
Depending on the database usage profile, the tempdb database may come in for intense and sustained usage. By providing dedicated disks for tempdb, the impact on other databases will be reduced while increasing performance for databases heavily reliant on it.
SYSTEM AND PROGRAM FILES:
SQL data files shouldn’t be located on the same disks as Windows system and program files. The best way of ensuring this is to provide dedicated disks for SQL Server data, log, backups and tempdb.
Note: Mount points are fully supported in Windows Server 2003 and 2008.
BI Informatica & SQL
Microsoft Certified Solutions Associate (MCSA)