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

Contains the amount of dirty memory at which a process generating disk writes will itself start writeback.

Note: dirty_bytes is the counterpart of dirty_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.

Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any value lower than this limit will be ignored and the old configuration will be retained.