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When creating files/directories in the overlay filesystem, we previously always called ensure_parent_dirs() to walk the entire path and verify each ancestor exists in the delta layer. This was expensive because it checked each path component against both delta and base layers. Now we skip this walk when the parent inode is already in the delta layer, since a directory can only be in delta if all its ancestors are too. This optimization eliminates the ensure_parent_dirs overhead for the common case of creating many files in the same directory.
When looking up a child in a directory that was created purely in delta (not copied from base), skip the base layer lookup entirely. Such directories can't have children in base, so the stat() syscall is wasted. This is detected by checking if the parent's delta inode has an origin mapping - if not, it was created in delta and never existed in base. Common case: npm creates node_modules/ in delta, then creates thousands of packages inside it. Previously each lookup would fall through to HostFS even though nothing could possibly exist there.
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