Frank, Please send your real code. In your snippet you have TFile f("subset.root"); this should be TFile f("subset.root","recreate"); Rene Brun Frank Winklmeier wrote: > > Hi, > > I want to select a subset of a tree which is split over several files and > save this subset to one new file. Everything works fine but the resulting > file size is much bigger than the sum of the input files. > > I start with: > > TChain *chain = new TChain("tree"); > chain->Add("data-*.root"); > > // there is data-1 to data-21, each about 13 MB > > TFile f("subset.root"); > TTree *subtree = chain->CopyTree("m0b0>0"); > f.Write(); > > The resuling size of subset.root is about 560 MB !!! > But the total size of the (unfiltered) input is only about 270 MB. > The compression level is 1 in all the files. > The selection criteria "m0b0>0" selects about 50% of all entries. > I am using RooT version 3.10-01 on RH9. > > How is that possible? Why is the compression so much worse for the subset? > > Thanks, > Frank
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