I have been working on a CD floppy control application on Redhat8. Now I am trying to find out how the system writes files to the floppy disk. I need to find out the daemons or programs which are watching and flushing the file system. Unfortunately I did not find them. Before I was doing this, I did:

killall magicdev
mv /sbin/update /sbin/update.old (disable bdflush)

I know that you may loss the file when you eject the floppy disk without umounting it. After mounting the floppy disk, the floppy light will be on whenever you copy a file to the floppy. But this somehow is not really writting data to the floppy disk. I notice that the light will be on again after few seconds and this time the file is really written to the floppy disk. I just want to know how the system does this.

Profoundly appreciate your help !

The kernel writes it, in fact. It may wait some time (uses buffers). You may look into the kernel source for details (but it may be a hard read). I suggest something about VFS (maybe "Linux kernel" by Bovet and Cesati from O'Reilly ).

Thank you very much, Mara.
I am wondering if the kernel calls other programs to do it or just do it by itself.

The kernel maintains the cache buffers itself. The logic of how it does so is complex and like Mara says would take an entire book to describe.

type sync to flush the buffers now.
it seems like if you mount the floppy sync, it
writes now, but i can't remember.

That's one of the things that must be done by kernel. Like scheduling, memory management etc.

Thank you again Mara.
My purpose is to know if it is the same floppy disk before the system(kernel) writes data on the disk. Linux is somehow weak to control floppy disk. According to your saying, I do not have better methords to do this except modifying kernel, am I right?

As long as you use the mount and umount commands the kernel has no problem keeping the floppy straight. The only time the floppy can get messed up is if you pull it out without doing a umount.

Incidently, the kernel uses the same logic for hard drives. The only way the hard drives can get messed up is if you power off or reboot without going through shutdown. Shutdown includes umount commands for the hard drives.

norman68, if you need to control the floppy directly (without mount/umount), you need to dig into the kernel and modify it. I don't see any other solution.