When using formatted I/O operations (typically operator<<(std::istream&...) this option controls the size of the in-memory buffer kept before a chunk is uploaded. Note that GCS only accepts chunks in multiples of 256KiB, so this option is always rounded up to the next such multiple.
Applications seeking optimal performance for downloads should avoid formatted I/O, and prefer using std::istream::write(). This option has no effect in that case.
