Comment by toast0
1 day ago
> Isn't it really obvious that a user space fs will always be slow, and especially slow with small files?
Small files seem like the perfect case for a user space fs... depending on what you mean by user space fs. If you mean interfacing with a FUSE (or similar) filesystem by using syscalls in your program to context switch into the kernel, then context switch to the userspace FUSE layer, then send it back to the kernel and then back to your program ... that will be especially slow with small files where date bytes per context switch is small. OTOH, if you mean a user space fs where your program has a built in filesystem it can access without context switching, then that will be of benefit ... especially if the files are small enough that you can pack multiple files into a single page.
If I got the article right, they were running FUSE inside the VM, and the VM's FUSE was talking to a process on the VM host (probably over virtual network). That can't be fast, not even theoretically.
Ever access to the fs had to go through two processes and two kernels, virtual networking, and probably even running on two different cores most of the time. That must be slow.