Size and Number of Logical-Log Files

After you know how much disk space to allocate for the entire logical log, you can make decisions about how many log files you want and what size.

When you think about the size of the logical-log files, consider these points:

• The minimum size for a logical-log file is 200 kilobytes.
• The maximum size for a logical-log file is essentially unbounded.
• If your tape device is slow, ensure that logical-log files are small enough to be backed up quickly.
• Smaller log files mean slower recovery because you potentially lose the last unbacked-up logical-log file if the disk that contains the logical-log files goes down.

Size of the Logical Log

Use the LOGSIZE configuration parameter to set the size of the logical log. It is difficult to predict how much logical-log space your database server system requires until it is fully in use. The following expression provides the minimum total-log-space configuration, in kilobytes, that is recommended:

LOGSIZE = (users * maxrows) * 512

Set users to the maximum number of users that you expect to access the database server concurrently. If you set the NETTYPE parameter, you can use the value that you assigned to the NETTYPE users field. If you configured more than one connection by setting multiple NETTYPE configuration parameters in your configuration file, sum the users fields for each NETTYPE, and substitute this total for users in the preceding formula.

Set maxrows to the maximum number of rows that you expect in the tables.

You can increase the amount of space devoted to the logical log as necessary and in several ways. The easiest way is to add another logical-log file. See Adding a Logical-Log File or Logslice.

Number of Logical-Log Files

When you think about the number of logical-log files, consider these points:

• You must always have at least three logical-log files.
• You should create enough logical-log files so that you can switch log files if needed without running out of free logical-log files.
• The number of logical-log files affects the frequency of logical-log backups.

The LOGFILES parameter provides the number of logical-log files. The LOGSIZE parameter provides the size of the logical-log files that are created when the database server initializes disk space. The database server administrator sets both of these configuration parameters in the ONCONFIG file. If all your logical-log files are the same size, you can calculate the total space allocated to the logical-log files as follows:

total logical log space = LOGFILES * LOGSIZE

If you add logical-log files that are not the size specified by LOGSIZE, you cannot use the (LOGFILES * LOGSIZE) expression to calculate the size of the logical log. Instead, you need to add the sizes for each individual log file on disk. For information on how to access the size of logical-log files, see Monitoring the Logical Log for Fullness.

For information on LOGSIZE, LOGFILES, and NETTYPE, see the chapter on configuration parameters in the IBM Informix: Administrator's Reference.