Making SD card corruption-proof

Solution 1:

You could use the block2mtd driver to use the transactional jffs2 or yaffs(2) filesystems you seem to be employing elsewhere for your SD card, which would solve your problem of data loss or filesystem corruption at poweroff.

Doing so might incur other problems, though. As the SD card is likely to have own mechanisms for wear levelling and sector remapping in place, these could interfere with jffs2's and yaffs' implementatons to do the very same, decreasing the lifespan or the performance of your SD card. If this is not an issue, it should be worth trying.

Solution 2:

Check whether the kernel you use supports flush and/or sync flag for vfat (it seems some versions ignore it, be careful!).

Or just do away with the filesystem altogether if everything can go into one file (as would be the case with a raw log stream!) or into a few fixed size files (use partitions ;)