根据这个内核文档，目录项永远不会存储在磁盘上。我相信目录是一个索引节点，其数据块仅包含文件名和索引节点块号（见下图）。但内核究竟是如何/home/foo/hello.c从/一路解析出来的呢？即内核如何知道哪个 inode 是/or /var？
Directory Entry Cache (dcache)
------------------------------
The VFS implements the open(2), stat(2), chmod(2), and similar system
calls. The pathname argument that is passed to them is used by the VFS
to search through the directory entry cache (also known as the dentry
cache or dcache). This provides a very fast look-up mechanism to
translate a pathname (filename) into a specific dentry. Dentries live
in RAM and are never saved to disc: they exist only for performance.
The dentry cache is meant to be a view into your entire filespace. As
most computers cannot fit all dentries in the RAM at the same time,
some bits of the cache are missing. In order to resolve your pathname
into a dentry, the VFS may have to resort to creating dentries along
the way, and then loading the inode. This is done by looking up the
inode.
