Copy on Write

It’s essentially a programming technique which is applicable in many contexts.
Filesystems
- CoW as an alternative way to do journaling (brtfs uses CoW vs journaling in ext4)
- You work on a copy. After you are done, the copy is then made the real file.
Memory Management
- When new process is created, it’ll just point the PTE to the resource, only when it’s written to it’ll make a copy of it to an actual physical address. See mmap.

Zero Copy

This is different from CoW , but I decided to put them in the same page anyway.

Optimizing Large File Transfers in Linux with Go
Linux kernel supports few syscalls with zero copy. Eg. sendfile (wrapper around splice)
It avoids kernel-userspace-kernel roundtrip.
sendfile is synchronous but can be async with io_uring when splice support is added.
- w sendfile the file content is still buffered, but in Kernel space using mmap
- sendfile doesn’t not use DMA directly but there can be DMA happeneing in the leaf
  - DEVICE_TO_MEMORY
    - DMA copy from disk drive to fd_in buffer
  - WHAT SENDFILE DOES
    - fd_in buffer passes the page reference to fd_out buffer via the pipe
  - MEMORY_TO_DEVICE
    - Ethernet card went directly to the fd_in buffer for the actual data by looking at the references in the fd_out buffer.