Why is [ssh,scp,rsync,etc] so slow, from a VM to another local machine on my gigabit local network?

Question

I have a VM A running the latest archlinux with QEMU, KVM and a Virtio network card. It is a buildbot and it needs to drop the result on another machine B on the local network.

The VM is started with this command:

qemu-system-x86_64 -net nic,model=virtio -net bridge,br=bridge0 -hda /some/place/disk.qcow2 -m 512 -enable-kvm

It is painstakingly slow, 9MB take 1h30 to complete, and it is mangled.

Any other traffic goes at the normal speed of the local network. Any other ssh traffic from A to another machine that is not B is normal. Traffic between all machines but A is normal.

Help? I don't understand this behaviour.

Answer 1

The Arch Linux Wiki has a specific page dedicated to this. It lists many different possibilities, and I do not know whether one/any/none applies to your VM. They go from the trivial (not enough memory, use KVM, and so on) to the exotic, disabling the cache or the barrier=0 option for mounted filesystems.

You may wish to try those you have not already implemented, and see whether any makes a difference, then report back.

I am a fellow Archer, and the best suggestion I can give you is always to look up the Arch Wiki first, because it is really well done.