Source Filesystem Change detection

Meta/Google both famously have large monorepos that wouldn’t fit on the disk of their developers anymore. By accessing the code through a custom-made VFS, the developers only need to transfer the source that they interact with to their workstations.

Zack is not aimed at repositories of that size. But it should handle large source repositories that can be stored on a typical Laptop and already reach a size where sequential file tree traversal is slow.

To speed up change detection, we should use OS dependent techniques to detect changes fast. E.g. on Linux, this is not easy. inotify does not work well with larger directory structures since one needs to watch all sub directories. fanotify on the other end cannot be constrained to a single directory.

Possible techniques include:

• Layering a passthrough VFS on top the source directory and constraining fanotify to the whole directory.
• Implementing a FUSE passthrough VFS that instruments all updates.