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0 pointsSkunkleton1mo ago0 comments

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.

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ars1mo ago

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.

userbinator1mo ago

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.

dezgeg1mo ago

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.

M95D1mo ago

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).

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