We've had to temporarily limit FTP access to mirrors.kernel.org due to high IO load.
We have recently upgraded our hardware in order to increase capacity -- 16TB was no longer nearly sufficient enough to host all the distro mirrors and archives. We chose larger but slower disks and offset the loss of performance by heavily utilizing SSD IO caching using dm-cache.
While it was performing very well, we have unfortunately run across an FS data corruption bug somewhere along this stack:
megaraid_sas + dm_cache + libvirt/virtio + xfs
We've temporarily removed dm-cache from the picture and switched to Varnish on top of SSD for http object caching. Unfortunately, as Varnish does not support FTP, we had to restrict FTP protocol to a limited number of concurrent sessions in order to reduce disk IO. If you are affected by this, simply switch to HTTP protocol that does not have such restrictions.
This is a temporary measure until we identify the dm-cache problem that was causing data corruption, at which point we will restore unrestricted FTP access.