MaxTreeSize and multiple trees

ROOT
1

Hi, in my analysis program I write the output on two trees inside the same root file. Two different routines write on each tree, and the tree have to be aligned (ie., entry N on both trees is relative to event N). My doubt is about MaxTreeSize. Suppose that routine 1 writes event N on tree 1, which makes the file reach the maximum dimension set by SetMaxTreeSize: what happens next? My guess is that file is closed, a new file named filename_1.root is created and then routine 2 writes event N on tree 2 in this new file. In the end I will have N events in tree 1 and N-1 events in tree 2 inside the original file. Likewise, the trees in the new file will have M-1 and M events respectively.
Is my guess correct? In this case, how can I handle the two files so that I would see two trees both with N+M elements? Is it sufficient to create a TChain?
Thanks

2

[quote] Likewise, the trees in the new file will have M-1 and M events respectively.
Is my guess correct? [/quote]Yes.

[quote]In this case, how can I handle the two files so that I would see two trees both with N+M elements? Is it sufficient to create a TChain?[/quote]Yes, you would need to create 2 chains (one for each TTree).

Alternatively, you can set the max tree size to a very large number and deal with just one file.

Cheers,
Philippe.

3

Thanks Philippe.

4

Hi Philippe,

Is this still the preferred method to handle this? Our use case could result in a file many TB in size, which is not ideal.

5

yes … but you ought to close the file and start a new file on your own. I.e. once you reach the threshold you are comfortable with (for example ‘so many entries’ or ‘so many GBs’). You need to:

  • Write the file

and either

  • Close the file (which delete the TTrees) and create a new file and recreate the TTree objects

or

  • Detach the 2 TTrees from the file (call mytree->SetDirectory(nullptr);)
  • Create the new file (and sub-directory if any)
  • Reattach the 2 TTrees to the new file (call mytree->SetDirectory(new_file_or_directory);)

Cheers,
Philippe.