Any such guarantees are only true for the OS and filesystem (ie the data has been transferred from main memory across whatever physical bus the disk is attached

Any such guarantees are only true for the OS and filesystem (ie the data has been transferred from main memory across whatever physical bus the disk is attached to ide/scsi/sata/spi/network/nvme, etc) In modern drives, the specific write operation may be queued up on the physical disk in volatile storage outside the control of the operating system. A flush can’t help you with this problem, flush is for the operating system to flush its cache to disk. This is where F_FULLFSYNC comes in. It is a request to the disk to finish flushing the volatile caches to nonvolatile storage. Some disks just ignore this request, claiming to be done before that is actually true. In these cases no userland or even kernel level programming tricks can save you from data loss, since you can’t actually know when the data becomes safe on the disk.