Re: Proposition to streamline our NAR collection to just zstd-compressed ones

["Ludovic Courtès" <ludo@gnu.org>]

Hello,

Maxim Cournoyer <maxim.cournoyer@gmail.com> skribis:

> It's been on my head for quite a bit of time (about 2 years, according
> to [0]), to streamline our offering of cached nars.  Letting go of gzip
> 2 years ago, along a more aggressive garbage collection policy allowed
> us to reduce our storage needs by at least 6.5 TiB.  I'm proposing to do
> the same with our lzip compressed nars, to let go of an additional 3.9
> TiB:

Those space savings would be welcome.

> The above suggests that zstd compressed nars are about 5% larger than
> the lzip ones, which is not big enough to justify carrying both, in my
> opinion.  In exchange for a little bit more bandwidth, users would have
> the nars decompressed much faster with less CPU overhead locally.

The difference is slightly higher, with lzip being 8% smaller, for a big
package like ungoogled-chromium or icecat:

--8<---------------cut here---------------start------------->8---
$ wget -qO- https://ci.guix.gnu.org/7n95j1zlnwzc44azjs7nj8givnzdfs87.narinfo|grep -B1 ^FileSize
Compression: lzip
FileSize: 85783483
--
Compression: zstd
FileSize: 92796393
$ wget -qO- https://ci.guix.gnu.org/prpjnnnhay0alanmkgjh66vfwjlb98kq.narinfo|grep -B1 ^FileSize
Compression: lzip
FileSize: 295991
--
Compression: zstd
FileSize: 323456
--8<---------------cut here---------------end--------------->8---

But yeah, even though adaptive compression selection on the client is a
minor improvement, whether it warrants the extra space is debatable.

> What do you think?  Should we go ahead and effect the following simple
> change for the Berlin build farm?
>
> modified   hydra/modules/sysadmin/services.scm
> @@ -683,7 +683,7 @@ to a selected directory.")
>                     ;; <https://lists.gnu.org/archive/html/guix-devel/2021-01/msg00097.html>
>                     ;; for the compression ratio/decompression speed
>                     ;; tradeoffs.
> -                   (compression '(("lzip" 9) ("zstd" 19)))
> +                   (compression '(("zstd" 19)))

No objection from me, but…

… an important consideration: zstd support was added in 1.3.0, released
in May 2021.

From experience we know that users on foreign distros rarely, if ever,
upgrade the daemon (on top of that, upgrading the daemon is non-trivial
to someone who initially installed the Debian package, from what I’ve
seen, because one needs to fiddle with the .service file to adjust file
names and the likes), and we can be sure that many are still running an
old daemon.  We spent a lot of time on user support after gzip
substitutes had been removed (‘guix substitute’ would just crash) and we
must avoid that.

(guix store) emits a warning when connecting to an “old” daemon, but
only for daemons older than 2018.  We could emit a warning based on
whether or not “builtin:git-download” is available, but maybe that’s too
early?

In addition to the warning, we should communicate in advance and make
sure our instructions on how to upgrade the daemon are accurate and
clear.

Thoughts?

Ludo’.