If TSS is started before JES via option SUB=MSTR in the COMMAND parmlib member then the JESMSGLG output dataset is created for the TSS started task once JES starts (provided PTF RO80352 is applied).
It is an improvement to have a JES Message Log but unfortunately it does not have everything we might need to resolve a severe TSS issue at IPL time. In particular, it only contains TSS messages written after
JES2 startup, so does not include messages about JES dataset allocations, for example.
Under the current architecture, TSS does not have a way to buffer these WTO messages before JES is up.
If the messages written between TSS startup and JES startup could be retained and written to the JES Message Log once JES is available, it would make the log much more useful for diagnostic purposes.