How does nix write in the read only store?

Help
1

Hello

This has been confusing me for a while, the nix store is mounted read-only, but yet Nix is able to add files in it. How does it work?

8 Likes
2

Afaik it’s basically the same way containers work. The mounts are different for the nix daemon compared to the rest of the system.

Technically, the store is mounted read-write, then bind-mounted read-only on top of itself. The nix daemon just doesn’t have that bind mount.

4 Likes
3

Yea this is correct:

$ mount | grep /nix/store
pyromancer/crypt/system/nix/store on /nix/store type zfs (rw,relatime,xattr,posixacl)
pyromancer/crypt/system/nix/store on /nix/store type zfs (ro,relatime,xattr,posixacl)

$ nsenter -m -t $(pgrep nix-daemon) mount | grep /nix/store
pyromancer/crypt/system/nix/store on /nix/store type zfs (rw,relatime,xattr,posixacl)
pyromancer/crypt/system/nix/store on /nix/store type zfs (rw,relatime,xattr,posixacl)

Two mounts, one for my actual /nix/store file system, one for the ro bind mount. But in the daemon’s mount name space, the latter has been remounted rw.

5 Likes
4

Thank you very much for your replies :slight_smile:

1 Like
5

Which system part ensures this mount? I.e. “who does the mount”? It’s not in fstab, nor in systemd mount target:

[motiejus@vno1-op5p:~]$ sudo systemctl cat nix-store.mount
No files found for nix-store.mount.

[motiejus@vno1-op5p:~]$ grep /nix/store /etc/fstab

[motiejus@vno1-op5p:~]$

I was about to raise a separate topic “where does it come from”, but found this instead. :slight_smile:

6
4 Likes