In V5R4, database files that were created on a release earlier than V5R4 are converted to eliminate a performance degradation that might occur due to Enhanced Hardware Storage Protection support.
Because database files (*FILE objects) consist of many machine interface (MI) objects, the time to convert them could be significant when an application is started the first time after V5R4 is installed. Also, because the MI objects will have their header size increased to 4K during the conversion, each MI object associated with a database file will grow in size by 4K (4096 bytes).
To spread the MI object conversions out over a period of time, the database
file conversion support is designed to be done when the database MI objects
are first accessed or used after V5R4 is installed. If you would like to evaluate
the impact that the database file conversions might have on one or more of
your applications and determine if you have enough time during your upgrade
window to take the necessary steps to eliminate the startup performance degradation
for your more critical applications, run the QDBFIMIC program that is included
in PTF SI18716 (if you are upgrading from V5R2) or PTF SI18615 (if you are
upgrading from V5R3). The cover letter for these PTFs (and
all other PTFs) is on the Support for iSeries Family 
Web site (www.ibm.com/servers/eserver/support/iseries/).
In the navigation bar, click Technical databases; then
click PTF Cover Letters.