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

[Maxim Cournoyer]

Hello Guix, and Happy New Year!

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:

--8<---------------cut here---------------start------------->8---
$ du -sh /var/cache/guix/publish/{lzip,zstd}
3.9T    /var/cache/guix/publish/lzip
4.1T    /var/cache/guix/publish/zstd

$ find /var/cache/guix/publish/lzip -name '*.nar' | wc -l
4484645
$ find /var/cache/guix/publish/zstd -name '*.nar' | wc -l
4461195
--8<---------------cut here---------------end--------------->8---

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.

Having our complete nars collection fit in around 4 TiB would also open
the door for simple rsync-based mirroring, which I have started working
on.

What do you think?  Should we go ahead and effect the following simple
change for the Berlin build farm?

--8<---------------cut here---------------start------------->8---
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)))
                    (cache-bypass-threshold cache-bypass-threshold)
                    (workers publish-workers)))
--8<---------------cut here---------------end--------------->8---

-- 
Thanks,
Maxim