When analyzing a large sample of events, it may be useful to write a subsample of events (which may have passed) in a new DST file, which then can be used later on and is looped through faster than the original one.

In your Python options-file, add:

from Configurables import InputCopyStream

InputCopyStream().Output = "DATAFILE='PFN:yourdatafile.dst' TYP='POOL_ROOTTREE' OPT='REC'"
YourSequence.Members.append(InputCopyStream())

If placed after a preselection, only the events fulfilling the preselection criteria, will be stored in the miniDST-file.

• The event is stored as a whole and with the same structure as in the original DST. E.g. If you store the particles of your preselection (using CombineParticles, for instance) in Phys/myPreSel, this structure will not be stored in your miniDST.
• DST files are big creatures (even if they are called “mini”). One event may be on the order of 500KB which will blow up your miniDST very quickly and resulting in a problem with your quota on lxplus.