@JP Vossen: I just looked through my system and noticed auth.log contains an hourly entry by pam_unix, (though mine are at :17 after) so it may be that's when pam renews its authentication, which probably causes the disk buffers to get flushed.
@JP Vossen: I just looked through my system and noticed auth.log contains an hourly entry by pam_unix, (though mine are at :17 after) so it may be that's when pam renews its authentication, which probably causes the disk buffers to get flushed.