zlacker

>>Galaxy+(OP)
The persistence strategy described here (mount -t msdos -o rw /dev/fd0 /mnt) combined with a bind mount to home is a nice clever touch for saving space.

I don't know if that's also true for data integrity on physical magnetic media. FAT12 is not a journaling filesystem. On a modern drive, a crash during a write is at best, annoying while on a 3.5" floppy with a 33mhz CPU, a write operation blocks for a perceptible amount of time. If the user hits the power switch or the kernel panics while the heads are moving or the FAT is updating, that disk is gone. The article mentions sync, but sync on a floppy drive is an agonizingly slow operation that users might interrupt.

Given the 253KiB free space constraint, I wonder if a better approach would be treating the free space as a raw block device or a tiny appended partition using a log-structured filesystem designed for slow media (like a stripped down JFFS2 or something), though that might require too many kernel modules.

Has anyone out there experimented with appending a tar archive to the end of the initramfs image inplace for persistence, rather than mounting the raw FAT filesystem? It might be safer to serialize writes only on shutdown, would love more thoughts on this.

>>Fivepl+95
Controversial position: journaling is not as beneficial as commonly believed. I have been using FAT for decades and never encountered much in the way of data corruption. It's probably found in far more embedded devices than PCs these days.

>>userbi+c8
If you make structural changes to your filesystem without a journal, and you fail mid way, there is a 100% chance your filesystem is not in a known state, and a very good chance it is in a non-self-consistent state that will lead to some interesting surprises down the line.

>>Skunkl+Kc
No, it is very well known what will happen: you can get lost cluster chains, which are easily cleaned up. As long as the order of writes is known, there is no problem.

>>userbi+uf
Better hope you didn't have a rename in progress with the old name removed without the new name in place. Or a directory entry written pointing to a FAT chain not yet committed to the FAT.

Yes, soft updates style write ordering can help with some of the issues, but the Linux driver doesn't do that. And some of the issues are essentially unavoidable, requiring a a full fsck on each unclean shutdown.

>>dezgeg+Hx
I don't know how Linux driver updates FAT, but if it doesn't do it the way DOS did, then it's a bug that puts data at risk.

1) Allocate space in FAT#2, 2) Write data in file, 3) Allocate space in FAT#1, 4) Update directory entry (file size), 5) Update free space count.

Rename in FAT is an atomic operation. Overwrite old name with new name in the directory entry, which is just 1 sector write (or 2 if it has a long file name too).

>>M95D+8p1
No, the VFAT driver doesn't do anything even slightly resembling that.

In general "what DOS did" doesn't cut for a modern system with page and dentry caches and multiple tasks accessing the filesystem without completely horrible performance. I would be really surprised if Windows handled all those cases right with disk caching enabled.

While rename can be atomic in some cases, it cannot be in the case of cross directory renames or when the new filename doesn't fit in the existing directory sector.