The Block Layer

In the previous article we followed a read() all the way down through the VFS: from the syscall, to the struct file, to ext4, and into the page cache. And there was exactly one moment where we skipped over the details. When the page cache missed — the bytes weren’t in memory yet — the filesystem had to actually go and fetch them, and all we said was that it “reads the disk, through the block layer.” That’s the moment we’re going to open up now.