Apologies for the delay! Without further knowledge, from the provided range I’d suggest the smallest value of a few tens of MB. Typically, ROOT files are read sparsely, so small block sizes should be beneficial. How big the read requests really are depends on the I/O buffer size of the application and on how the file was written. If you have sample applications, TTreePerfStats can be used to determine the key figures. Perhaps @pcanal has more experience specifically with ZFS.