file: /proc/sys/vm/dirty_background_bytes
variable: vm.dirty_background_bytes
Official reference

Contains the amount of dirty memory at which the background kernel flusher threads will start writeback.

Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only one of them may be specified at a time. When one sysctl is written it is immediately taken into account to evaluate the dirty memory limits and the other appears as 0 when read.