Questions regarding TTree and their NameCycles

[pcanal]

Hi,

You may want to re-read the User’s Guide chapter on I/O and in particular the discussion about ‘cycles’

When that tree became larger than its buffer size it was written to the file and ROOT continued on writing to a tree called myTree;2. Is this correct?

Sorta … When the number of entries reached the one given by ‘AutoSave’, the meta-data is flushed to the disk. A the previous version is kept around in case the writing of the new fail (so that most of the data is still readable even in case of crash). The cycles are backups.

Is it always guaranteed that myTree;1 will be a subset of myTree;2?

Yes. ;1 is an older snapshot of the meta data.

I would expect the file size to be reduced. But this doesn’t seem to be the case! Why?

The delete only remove the meta data and not the data (as it might be associated with other cycles) and thus the amount freed is small. In addition, this size freed is likely to be in the middle of the file and thus the file can not be shrunk as shrinking it would requires moving the other part (and updating the meta data pointing to them).

Presumably then ROOT is smart enough to know it should always pick the tree with the greatest namecycle? Is this the default behavior?

Yes.

5. If my understanding is all correct it really seems that a more intuitive behavior for writing the file would be to delete myTree;1 and leave me only with myTree;2. Why does this not happen? Why is myTree;1 still kept as a key in the final file?

See the User’s Guide for a complete answer and my previous answer for the short version.

Cheers,
Philippe.