[gighi]

OP here

That's correct, I should have included a chart explicitly measuring the I/O activity done by the two containers, but I can assure you there was literally no I/O activity, a dozen open files per second is a very negligible throughput. The bottleneck was solely in the cache.

[pvg]

Yep, the old version of this was 'why is tar/find/rsync/etc hosing my server even when I've [io]niced it to hell'. Except now (as everything else) with containers!