we want to change the ext4 mode to writeback,
how should i do that ?
/etc/fstab ?
if so,
what's the exact line pls ?

Thank you !

Why is your ext4 filesystem on a journey?

You can mount an ext4 filesystem with the data=writeback option. The default seems to be data=ordered

This is what i was looking for.

we use SD card in the embedded system, was trying to make sure the data won't get corrupted.

But seems it causes plenty of delay when write data back to SD.
E.g. when we soft reset the board, we try to close all files, was hoping to save all data to SD.
After wait ~5 seconds, we poweroff.
But a lot of times when we boot up, the files are missing.
So our random guess is the ext4 is not writing back to SD and complete its task on time.

Thanks for your reply and we'll try it out.

This is what writeback does:
So after 5 seconds, if all data hasn't been written yet, you could end up with old versions of the files. You could run the sync command before powering off. That should already happen, unless the embedded system can't shut itself down.

You really need to dismount the filesystem before doing a "reset". You might consider making the "reset" button a software event and do a forced shutdown (there should already be a power failure trap that would do the same thing).

Even with data "writeback", doesn't guarantee that the metadata has been written - and until that metadata is recorded the "data" is not really accessible to the file after a reset. NOTE: Here I'm meaning that the disk cache may have the data - as far as the system is concerned, the data is written--- but a hardware reset could cause the disk to also reset, causing a loss of the cache before it is actually on disk. This really depends on the disk drive involved.

Thanks for the info.
we've tried sync(), umount before, still have issues, all files in the folder with frequent I/Os could all get lost.
Let's see how writeback works.

I would re-partition, then reformat the SD card. And consider it likely faulty.