Re: [ROOT] Size of root file after writing a subset of a tree

From: Frank Winklmeier (frank.winklmeier@colostate.edu)
Date: Tue Jan 27 2004 - 18:45:11 MET 

Hi Rene,

sorry about that. Here it is:

TChain *chain1 = new TChain("xtree");
chain1->Add("/hep42/frank/root/xjpsi-*.root");

TFile f1("/hep42/frank/root/xjpsi-B0.root","recreate");
TTree *xtree0 = chain1->CopyTree("m0B0>0");
f1.Write();

Frank


On Tue, 27 Jan 2004, Rene Brun wrote:

> 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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