[asdbffg]

Using O_APPEND does not imply, that kernel needs to purge the pages from cache ASAP, does it? Removing pages from cache may be expensive operation by itself, so I presume, that it is avoided by default.

More importantly, if the disk can not catch up, the log data is going to end up waiting in page cache anyway (typical case of bufferbloat). Linux kernel does not have telepathic abilities to balance needs of crazy logger and other applications in system, so without resolving underlying issue (bufferbloat), those writes would take up too much cache, potentially bringing down disk performance of other applications.

fadvise() may schedule quicker eviction, effectively acting as syscall version of vm.dirty_ratio. Of cause, that does not resolve the problem, — just moves it to different layer. The real solution is either

1) blocking the apps until their logs are fully written (for example, by using O_DIRECT)

2) showing those apps middle finger and throwing away some of their logs (AFAIK, this is occasionally done by syslog).