[Skunkleton]

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.

[ars]

FAT has two allocation tables, the main one and a backup. So if you shut it off while manipulating the first one you have the backup. You are expected to run a filesystem check after a power failure.

[userbinator]

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.

[dezgeg]

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.

[M95D]

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).

[dezgeg]

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.

[M95D]

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

Which driver? DOS? FreeDOS? Linux? Did you study any of them?

> 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.

That's a "move". Yes, you would need to write 2-6 sectors in that case.

For short filenames, the new filename can't not fit the directory cluster, because short file names are fixed 8.3 characters, pre-allocated. A long file name can occupy up to 10 consecutive directory entries out of the 16 fixed entries each directory sector (512B) has. So, an in-place rename of a LFN can write 2 sectors maximum (or 1KB).

Considering that all current drives use 4KB sectors at least (a lot larger if you consider the erase block of a SSD) the rename opearation is still atomic in 99% of cases. Only one physical sector is written.

The most complicated rename operation would be if the LFN needs an extra cluster for the directory, or is shorter and one cluster is freed. In that case, there are usually 2 more 1-sector writes to the FAT tables.

Edit: I corrected some sector vs. cluster confusion.