Technical Blog Post
What are the .EXP files showing up in active log path or, overflow log path
When the database recovery happens DB2 retrieves the transaction log files and use those for rollforward.
If the logs are compressed ones then it first DB2 uncompress those to same name except .EXP extension before using that.
In-place uncompress also possible if logs are uncompressed manually using internal log uncompress tool.
This is one reason, the DB2 cannot use a compressed log in a read-only filesystem as it cannot write the .EXP file there.