bug#24937: "deleting unused links" GC phase is too slow

[John Kehayias via Bug reports for GNU Guix <bug-guix@gnu.org>]

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Hello,

A little late, but wanted to add my results here, from before the commit was made. I had reported some of this on IRC before and had some outlying results. Since then I finally did some generation deleting and gc-ing, though perhaps still have a bit of cruft. I've been on core-updates-frozen for a while, so keeping a lot from before making the branch switch, as well as lots of stuff piling up in trying to fix things or trying out changes in core-updates-frozen.

Anyway, attached are the plots from the above code. Running

ls -1A /gnu/store/.links | wc -l

showed 15776256 links at the time. Still quite a bit I think, but I've had 1.5-2x as much in the past, easily. (This had caused some earlier warnings on ext4 and enabling large_dir, which will make a system unbootable due to Grub not being up to speed on this old feature. I'm now on btrfs.)

John

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