By default, every time you start a TDI server, the old ibmdi.log will be renamed to ibmdi.log.1, the previous ibmdi.log.1 will be named ibmdi.log.2, and so on.
So suppose you start a TDI server. It will begin logging to ibmdi.log.
Then you start a new TDI server, and the first TDI server will now log to ibmdi.log.1, while the new TDI server will log to ibmdi.log.
If you now start yet another TDI server, that TDI server will log to ibmdi.log, while the previous TDI server will log to ibmdi.log.1, and the first TDI server will log to ibmdi.log.2.
You now have 3 different TDI servers running, logging to different files.
If a rollover due to file size happens, the TDI server that have to do the rollover will begin logging to ibmdi.log, while all the other TDI servers will have the log file name bumped by one.
You can change the behavior by modifying etc/log4j.properties. The default appender is the TDIRollingFileAppender, which behaves like this:
## TDIRollingFileAppender behaves like org.apache.log4j.RollingFileAppender, except that it also
## rolls over log files when opening the log files, if Append=false.
## The RollingFileAppender rolls over log files when they reach a certain size
## specified by the MaxFileSize parameter, and the number of backup files is specified by MaxBackupIndex