Constant READ_BUFFER_SIZE 

pub(crate) const READ_BUFFER_SIZE: usize = _; // 8_192usize

The most a read of a container I/O stream takes at once. The read loops read into the scratch buffer of their thread and copy each chunk out of it, see with_scratch, so an idle container holds no read buffer. 8 KiB halves the reads and log writes of 4 KiB for bulk output, the log files start the buffer of each chunk with as much, see LogFile::BUF_CAPACITY.