bug#51787: GC takes more than 9 hours on berlin

[Mathieu Othacehe]

Hey,

> OK, thanks. Looks it just finished removing the trash directory
> content. I started a GC process from my session to monitor it closely.

Daily GC recap:

* The GC process I started yesterday, did collect 5.5TiB in
  approximately 24 hours, that are now in the /gnu/store/trash
  directory.
  
* The /gnu/store/trash directory contains 288910 entries. If those items
  are removed at the same rate than on the previous days, it will take
  days/months to delete them all.

* I noticed that the upstream Nix GC process can now operate without
  locking. I think it shouldn't be too hard to port it to our fork or
  maybe rewrite the process in Guile while we are at it.

  That will not fix the slow hard-drives issues though.

Thanks,

Mathieu