Re: Building and caching old Guix derivations for a faster time machine

[Maxim Cournoyer]

Hi,

Ludovic Courtès <ludo@gnu.org> writes:

> Hi,
>
> Ricardo Wurmus <rekado@elephly.net> skribis:
>
>> to me the biggest downside of using “guix time-machine” is that it has
>> to do a lot of boring work before the interesting work begins.  The
>> boring work includes building Guix derivations for the given channels,
>> most of which have long been collected as garbage on ci.guix.gnu.org.
>>
>> It would be helpful, I think, to more aggressively cache these
>> derivations and their outputs, and to go back in time and build the
>> derivatinons for past revisions of Guix.  I would expect there to be a
>> lot of overlap in the produced files, so perhaps it won’t cost all that
>> much in terms of storage.
>>
>> What do you think?
>
> I agree.  The ‘guix publish’ TTL¹ at ci.guix was increased to 180 days
> following <https://issues.guix.gnu.org/48926> in 2021.  That’s still not
> that much and these days and right now we have 84 TiB free at ci.guix.
>
> I guess we can afford increasing the TTL, probably starting with, say,
> 300 days, and monitoring disk usage.
>
> WDYT?

While the 84 TiB we have at our disposal is indeed lot, I'd rather we
keep the TTL at 180 days, to keep things more manageable for backup/sync
purposes.  Our current TTL currently yields 7 TiB of compressed NARs,
which fits nicely into the hydra-guix-129 10 TiB slice available for
local/simple redundancy (it's still on my TODO, missing the copy bit).

I've been meaning to document an easy mirroring setup for that
/var/cache/guix/publish directory, and having 14 TiB instead of 7 TiB
there would hurt such setups.

Perhaps a compromise we could do is drop yet another compression format?
We carry both Zstd and LZMA for Berlin, which I see little value in; if
we carried only ZSTD archives we could probably continue having < 10 TiB
of NARs for a TTL of 360 days (although having only 3.5 TiB of NARs to
sync around for mirrors would be great too!).

What do you think?

-- 
Thanks,
Maxim