Should I enable nix.autoOptimiseStore on my laptop?

das-g · March 29, 2021, 11:05pm

Back in 2012, Nix’ autoOptimiseStore setting was set to false to avoid a performance problem. That seems to be what it’s still defaulting to today:

github.com

NixOS/nix/blob/1581c3e8ef31569b54cfb71e7fbffcc18af9eedf/src/libstore/globals.hh#L360

    
      
          
          
      Keeping derivation around is useful for querying and traceability
                (e.g., it allows you to ask with what dependencies or options a
                store path was built), so by default this option is on. Turn it off
                to save a bit of disk space (or a lot if `keep-outputs` is also
                turned on).
              )",
              {"gc-keep-derivations"}};
          
          
Setting<bool> autoOptimiseStore{
              this, false, "auto-optimise-store",
              R"(
                If set to `true`, Nix automatically detects files in the store
                that have identical contents, and replaces them with hard links to
                a single copy. This saves disk space. If set to `false` (the
                default), you can still run `nix-store --optimise` to get rid of
                duplicate files.
              )"};
          
          
Setting<bool> envKeepDerivations{
              this, false, "keep-env-derivations",

The corresponding NixOS option nix.autoOptimiseStore seems to have also defaulted to false ever since it was introduced in
https://github.com/NixOS/nixpkgs/commit/68c6d90417c5fc0d75c140e5e51e4f026cd6edff

Is there still a good reasons for these defaults today? On my laptop, where I’ve made it a habit to run

nix-store --optimize

after every other or third nixos-rebuild, is there anything to consider before setting nix.autoOptimiseStore to true instead of continuing to manually invoke nix-store --optimize every now and then?

wmertens · March 30, 2021, 4:38am

I have it on without any apparent issues.

ElvishJerricco · March 30, 2021, 7:16am

I have it enabled and it saves 8818.67 MiB. I’ve never noticed any practical drawback from it.

NobbZ · March 30, 2021, 7:34am

I have it active, and I have to be honest, there is a noticeable slow down on write operations in the store.

Getting a nixpkgs “checkout” into the store takes about 30 seconds when I have nix.autoOptimiseStore enabled, though its 5 to 10 when disabled.

wmertens · April 21, 2021, 12:02pm

Note that it needs to checksum the files but even then that seems like a huge difference. Either the checksumming is super slow or your fs can’t handle the large directory of hardlinks. What fs are you using, and SSD or HDD?

NobbZ · April 21, 2021, 12:05pm

ZFS with compression and dedup for that dataset. Though I have activated dedup only recently, it was not active when wrote my last post.

Drive is an SSD (Samsung QVO, 1 TB)

wmertens · April 21, 2021, 12:21pm

Hmm, that’s quite a slow drive for random writes. Perhaps the hardlinks are causing way more fragmented writes? There are a ton of small files in nixpkgs.

In any case, doesn’t seem like an easy-to-diagnose issue.

wmertens · April 21, 2021, 12:56pm

I checked and nix doesn’t seem to combine the checksumming with other operations. So it causes extra reads and writes.