If you're upgrading to V2R1M1 directly from V1R3M0 and you have installed on your system the PTF that allows object compression on V1R3M0, you may be in for some trouble.
In the Memo To Users provided with V2R1M1, under 126.96.36.199. Database File Conversion, IBM recommends that you execute the following command to cause a "first access" file conversion to take place for all files, after upgrading to V2R1M1:
DSPFFD FILE(*ALL/*ALL) + OUTPUT(*OUTFILE) + OUTFILE(QTEMP/OUT)
However, this command will terminate after several hours with an MCH0601 error while working on some QSYS files. It seems there are six display files in QSYS and four panel groups in QHLPSYS left over from V1R3M0 that didn't get deleted. A PTF at V2R1M0 that fixed the problem was overlooked by V2R1M1.
To solve the problem, IBM Level 2 recommends you do the following, before running the DSPFFD command:
DLTF FILE(QSYS/QDDSPAAU) DLTF FILE(QSYS/QDDSPACC) DLTF FILE(QSYS/QDDSPDA) DLTF FILE(QSYS/QDDSPSRV) DLTF FILE(QSYS/QDFRUDP) DLTF FILE(QSYS/QDSUMDP) DLTPNLGRP PNLGRP(QHLPSYS/QHOE) DLTPNLGRP PNLGRP(QHLPSYS/QHSR1) DLTPNLGRP PNLGRP(QHLPSYS/QHSS1SYN) DLTPNLGRP PNLGRP(QHLPSYS/QHWPCMD)