从kafka论文里摘的：
In addition we optimize the network access for consumers. Kafka
is a multi-subscriber system and a single message may be
consumed multiple times by different consumer applications. A
typical approach to sending bytes from a local file to a remote
socket involves the following steps: (1) read data from the storage
media to the page cache in an OS, (2) copy data in the page cache
to an application buffer, (3) copy application buffer to another
kernel buffer, (4) send the kernel buffer to the socket. This
includes 4 data copying and 2 system calls. On Linux and other
Unix operating systems, there exists a sendfile API [5] that can
directly transfer bytes from a file channel to a socket channel.
This typically avoids 2 of the copies and 1 system call introduced
in steps (2) and (3). Kafka exploits the sendfile API to efficiently
deliver bytes in a log segment file from a broker to a consumer.