Re: “guix gc”, auto gcroots, cluster deployments

[Ricardo Wurmus <rekado@elephly.net>]

Ludovic Courtès <ludovic.courtes@inria.fr> writes:

> Hi!
>
> Ricardo Wurmus <rekado@elephly.net> skribis:
>
>> There are two problems here:
>>
>> 1) I don’t think “guix gc --list-dead” (or “--list-live”, or 
>> more
>> generally “findRoots” in nix/libstore/gc.cc) should delete 
>> anything.  It should just list and not clean up.
>
> Maybe ‘findRoots’ could populate the list of stale roots and 
> it’d be up
> to the caller to decide whether to delete them or not?

Yes, this would be better.  It already does this for links whose 
targets exist but cannot be read.

>> 2) For cluster installations with remote file systems perhaps 
>> there’s
>> something else we can do to record gcroots.  We now have this
>> excursion into unreadable space because we use a symlink, but 
>> the
>> start ($localstatedir/gcroots/auto) and endpoints 
>> (/gnu/store/…) are
>> both accessible by the daemon.  Since these intermediate 
>> locations are
>> tied to user accounts, could we not store them in a per-user
>> directory?
>>
>> This problem does not exist for user profiles, because the link 
>> in
>> unreadable home directories is not all that important; it 
>> merely 
>> points to $localstatedir, which is always readable by the
>> daemon. Perhaps we could do the same for temporary roots and 
>> let
>> *users* decide when to let go of them by giving them a command 
>> to
>> erase the important links in $localstatedir.
>>
>> So instead of having a link from
>> /gnu/var/guix/gcroots/auto/8ypp8dmwnydgbsgjcms2wyb32mng0wri to 
>> /home/me/projects/mrg1_chipseq/.guix-profile-1-link pointing to
>> /gnu/store/ap0vrfxjdj57iqdapg8q83l4f7aylqzm-profile, we would 
>> record
>> /var/guix/profiles/per-user/me/auto/8ypp8dmwnydgbsgjcms2wyb32mng0wri 
>> pointing to 
>> /gnu/store/ap0vrfxjdj57iqdapg8q83l4f7aylqzm-profile, and
>> then point /home/me/projects/mrg1_chipseq/.guix-profile-1-link 
>> at
>> that.  Yes, removing
>> /home/me/projects/mrg1_chipseq/.guix-profile-1-link would no 
>> longer free up the profile for garbage collection, but removing
>> $(readlink /home/me/projects/mrg1_chipseq/.guix-profile-1-link) 
>> would.
>
> Yes, but how would per-user/me/auto/* be cleaned up?

Yeah, that’s an open question.

I get the appeal of having these things be cleaned up 
automatically when the link disappears, but if we added this extra 
layer of indirection for cluster deployments this would become 
manual.

Can we make this configurable perhaps…?  On my cluster 
installation I’d rather have a cron job to erase the stuff in 
per-user/me/auto/* on my own terms, than to have “guix gc” fail to 
resolve links and consider it all garbage.

-- 
Ricardo