In one embodiment, an extent store layer of a storage inputoutput stack executing on one or more nodes of a cluster manages efficient logging and checkpointing of metadata. A projection is formed on an upper surface of the lower frame, wherein a plurality of notched recesses are formed on an outer peripheral surface of the projection. The resulting plexifilamentary film-fibril sheet material has increased breathability with no decrease in liquid barrier. The terminal voltages of all monomer batteries are completely equal when charging is finished, and no equalized charging is required.