How does nix write in the read only store?

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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?

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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.

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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.

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Thank you very much for your replies :slight_smile: