tags : Algorithms, Systems, Filesystems
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.
The case of sendfile
- Optimizing Large File Transfers in Linux with Go
- Linux kernel supports few syscalls with zero copy. Eg.
sendfile
(wrapper aroundsplice
) - It avoids kernel-userspace-kernel roundtrip.
sendfile
is synchronous but can be async with io_uring whensplice
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.
- DEVICE_TO_MEMORY
- w