Estimating the Size of the Physical Log

If your workload is update intensive, you can use the following formula to calculate the size of the physical log where PHYSFILE equals the physical-log size:

PHYSFILE = (users * max_log_pages_per_crit_sect * 4 * pagesize) / 1024

Variable in Formula

Description

users

The maximum number of concurrent user threads for which you can obtain an estimate when you execute onstat -u during peak processing. The last line of the onstat -u output contains the maximum number of concurrent user threads.

If you set the NETTYPE parameter, sum the values specified in the users field of each NETTYPE parameter in your ONCONFIG file.

max_log_pages_per_crit_sect

Maximum number of pages that the database server can physically log in a critical section. Substitute one of the following values:

5 if you do not use R-tree indexes

10 if you use R-tree indexes

4

Necessary factor because this part of the formula represents only 25 percent of the physical log:

users * max_log_pages_per_crit_sect

pagesize

System page size in bytes that you can obtain with oncheck -pr

1024

Necessary divisor, because you specify the PHYSFILE parameter in units of kilobytes

This formula is based on how much physical logging space the database server needs in a worst-case scenario. This scenario takes place when a checkpoint occurs because the log becomes 75 percent full. This size might be too small or large for your actual workload or configuration.

If you are using simple large objects in a dbspace in a database without logging, substitute the size of the most-frequently occurring simple large object in the dbspace for the maximum log pages per critical section.

For more information on monitoring and tuning the physical log, refer to the chapter on configuration effects on I/O utilization in your IBM Informix Dynamic Server Performance Guide.