- A middleware change slowed a file service by disabling sendfile(2) optimization in Go.
- Go's io.Copy uses sendfile(2) for *os.File sources via *net.TCPConn.ReadFrom, avoiding user-space copies.
- Wrapping *os.File in a custom io.Reader loses sendfile optimization, increasing syscalls and CPU usage.
- io.LimitReader preserves the fast path as Go recognizes it, unlike opaque wrappers.
- socket-to-socket proxies use splice(2) for zero-copy transfers between *net.TCPConn instances.
- strace helps verify fast path usage with sendfile/splice counts versus read/write syscalls.
- Avoid wrapping io.Reader/Writer between file/socket to maintain zero-copy optimizations in Go.