Help require in Memory resident feature of Informi

In our application we would like to load one index into memory to avoid continuous IO on disk. This index is used continuously by all the queries. Our assumption is that after loading this index into memory we will save on disk IO and hence will get improvement in the performance. We are following below mentioned step. - Modify RESIDENT parameter in onconfig file to 1 - executed "SET Index <index name> Memory_resident" statement - executed SQL query which will use only this index. After this point we executed several queries on this index but every time it was doing disk IO (We are using onperf utility to check IO in the respective dbspace during executing any query.) The index size is around 20Mb. Our expectation was, it should read data from memory. Would you please check whether we are missing something or our understanding of "set residency" command is wrong. Thanks Raj