See https://gerrit.wikimedia.org/r/#/c/operations/dumps/+/585264/
Not all files were being returned for prefetch; we should have unit testing to cover the case where multiple files are required for a pagerange that does not correspond to any of the previous run's output files.

This likely contributed to the slowness of the dump runs in March, although a look at the % prefetched in the logs didn't show anything too awful.