Making KIO copy many files fast

There is a bug in KDE's bug tracker that is almost as old as some of our contributors: bug 342056 , "Ridiculously slow file copy (multiple small files)", reported by Alexander Nestorov back in 2014. The report is blunt: copying a 15 GB…

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There is a bug in KDE's bug tracker that is almost as old as some of our contributors: bug 342056 , "Ridiculously slow file copy (multiple small files)", reported by Alexander Nestorov back in 2014. The report is blunt: copying a 15 GB folder of roughly 3 million small files took 5 to 10 hours in KDE, versus about 20 minutes with rsync . One commenter summed up the mood:

I generally use cp and rsync instead of dolphin for anything more than a few files.

That bug has been stuck in my head for a while, so this post is about finally fixing it. I have been working on KIO's copy path, and I want to walk through where the time was going, some history that explains why, and show numbers across KIO versions including a plain cp for reference.

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