OS disk cache usage

Hi All,
A partner is asking how Vertica uses the Linux file system cache (not the Verrica internal cache files). They are refering to instructions on how to clear it in the Notes section on https://my.vertica.com/docs/8.1.x/HTML/index.htm#Authoring/SQLReferenceManual/Functions/VerticaFunctions/CLEAR_CACHES.htm
It seems to me that Linux just caches writes to disk in memory and Vertica has nothing to do with it. Or is Vertica somehow aware of what is going on in the OS file system cache? Does Vertica write directly to disk or through this cache?