> Among other things, say the unlinked file is large enough that we
> can't cache all its block metadata in-core.  Then there's no way to
> "undelete" it as we can't remember where all the blocks are.
> Remember a 4 TB ufs2 file has around 1 GB of block pointer data.

Yes, but you don't trash most of it when you delete it.

The inode is zeroed; that you do need to keep cached.  But the indirect
blocks you just mark as free; you don't scribble on them.  Provided
they haven't been overwritten (which is assumed when you go RO), and
provided you still have everything the inode had, you're fine.

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