Disk drives operate much more slowly than modern processors or RAM. Often, simply waiting for the disk to read or write pages is the reason that a database server is slow.
You may improve database performance by putting different physical database files on different physical devices. For example, while one disk drive is busy swapping database pages to and from the cache, another device can be writing to the log file.
Notice that to gain these benefits, the devices must be independent. A single disk, partitioned into smaller logical drives, is unlikely to yield benefits.
SQL Anywhere uses four types of files:
transaction log (.log)
transaction log mirror (.mlg)
temporary file (.tmp)
The database file holds the entire contents of your database. A single file can contain a single database, or you can add up to 12 dbspaces, which are additional files holding portions of the same database. You choose a location for the database file and dbspaces.
The transaction log file is required for recovery of the information in your database in the event of a failure. For extra protection, you can maintain a duplicate copy of the transaction log in a third type of file called a transaction log mirror file. SQL Anywhere writes the same information at the same time to each of these files.
Placing the transaction log mirror file (if you use one) on a physically separate drive helps protect against disk failure, and SQL Anywhere runs faster because it can efficiently write to the log and log mirror files. To specify the location of the transaction log and transaction log mirror files, use the Transaction Log utility (dblog), or the Change Log File Settings Wizard in Sybase Central. See Transaction Log utility (dblog), and Change the location of a transaction log.
The temporary file is used when SQL Anywhere needs more space than is available to it in the cache for such operations as sorting and forming unions. When the database server needs this space, it generally uses it intensively. The overall performance of your database becomes heavily dependent on the speed of the device containing the temporary file.
If the temporary file is on a fast device, physically separate from the one holding the database file, SQL Anywhere typically runs faster. This is because many of the operations that necessitate using the temporary file also require retrieving a lot of information from the database. Placing the information on two separate disks allows the operations to take place simultaneously.
Choose the location of your temporary file carefully. The location of the temporary file can be specified when starting the database server using the -dt server option. If you do not specify a location for the temporary file when starting the database server, SQL Anywhere checks the following environment variables, in order:
If an environment variable is not defined, SQL Anywhere places its temporary file in the current directory for Windows, and in the /tmp directory for Unix.
If your computer has enough fast devices, you can gain even more performance by placing each of these files on a separate device. You can even divide your database into multiple dbspaces, located on separate devices. In such a case, group tables in the separate dbspaces so that common join operations read information from different dbspaces.
When you create all tables or indexes in a location other than the system dbspace, the system dbspace is only used for the checkpoint log and system tables. This is useful if you want to put the checkpoint log on a separate disk from the rest of your database objects for performance reasons. To create base tables in another dbspace change all the CREATE TABLE statements to use the IN DBSPACE clause to specify the alternative dbspace, or change the setting of the default_dbspace option before creating any tables. Temporary tables can only be created in the TEMPORARY dbspace. See default_dbspace option, and CREATE TABLE statement.
For more information about the default dbspace for base and temporary tables, see Using additional dbspaces.
A similar strategy involves placing the temporary and database files on a RAID device or a stripe set. Although such devices act as a logical drive, they dramatically improve performance by distributing files over many physical drives and accessing the information using multiple heads.
You can specify the -fc option when starting the database server to implement a callback function when the database server encounters a file system full condition. See -fc dbeng12/dbsrv12 server option.
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