Re: Large Files .. separate files

Rene Brun (Rene.Brun@cern.ch)
Tue, 10 Feb 1998 10:34:24 +0100

Thomas Hadig wrote:
>
> Dear root users,
>
> i have a question concerning large data volumes.
>
> At H1 our data is split in approx 200 MB files which are stored on tape
> and can be retrieved via fpack. Is it possible to do it in a similar way
> in root ?
>
> Of course it would be possible to write N events to a files, so that the
> files has an approximate size of 200 MB, but if i want to plot one variable
> i would have to copy each file from tape (which is the slowest step) and
> add up the distributions calculated for each file. It would be more clever,
> if one would have one branch of the TTree in one file, so that i
> would have to stage one file only (and maybe the files containing the
> tree structure). Is there a way in root to make this automatically ?
> (i.e. opening a TFile with a TTree containing several branches results in
> serveral files, each of them containing only a few branches ?)
>

We will provide this feature. This is quite easy to implement.
One possible user interface could be.
After the TBranch *branch = TTree::Branch call, add an additional
statement
branch->SetFile(file_name);
When doing the TTree::Write at the end, one must probably write
the branch definition to the file containing the branch buffers
such that this file can be used independently without accessing
the file containing the original Tree definition.
Comments are welcome.

Rene Brun