> ... Regular filesystems will buffer, and when the system is low on memory it will flush buffers ...
That is the bit you are missing. Unwritten filesystem data is regularly flushed. The flush interval is often around 5 seconds. Lookup "pdflush" to get the gist, although things have changed since then. Same with laptop mode.
Quite simply if a file is created and lives for at least N seconds then there will be disk activity irrespective of memory pressure. N is 5, perhaps up to 30 seconds in normal use.
Even if the file contents aren't fully flushed, metadata is.
That is the bit you are missing. Unwritten filesystem data is regularly flushed. The flush interval is often around 5 seconds. Lookup "pdflush" to get the gist, although things have changed since then. Same with laptop mode.
Quite simply if a file is created and lives for at least N seconds then there will be disk activity irrespective of memory pressure. N is 5, perhaps up to 30 seconds in normal use.
Even if the file contents aren't fully flushed, metadata is.