Network lagging, how to discover root cause?

postcd

Hello, on the dedi server, the network lagging (yum running 30 minutes already, downloads running few kb/s. It ran 30Mb/s but when i launched second rsync at same time it started lagging and even after terminating rsyncs, the network lagging.

im connecting from Czech republic, at same time on another server (also USA based), mtr goes quick and without issues.

I want to ask, how to discover where is the issue. I mean if it is right on this dedicated server, how to discover it. thx

postcd

UPDATE:
When i run two rsync in two SSH session windows it is OK for some minutes, but when i run one of the rsync in Linux "screen", it start immediatelly lagging and even after terminating transfers networking is lagging untill server restart. that is strange
by "lagging", i mean transfer is a few tens of kB/s and very slow response on keyboard inputs

Maounique

It is either throttling to the bone or a faulty card with buffers dead or something along those lines.
To be on the safe side try this:

echo 'net.core.wmem_max=12582912' >> /etc/sysctl.conf
echo 'net.core.rmem_max=12582912' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_rmem= 10240 87380 12582912' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_wmem= 10240 87380 12582912' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_window_scaling = 1' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_timestamps = 1' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_sack = 1' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_no_metrics_save = 1' >> /etc/sysctl.conf
echo 'net.core.netdev_max_backlog = 5000' >> /etc/sysctl.conf

then

sysctl -p

Make sure you have a copy of the old sysctl.conf if there is no improvement, also, the first 4 lines are very safe, the others might create issues in some conditions so it would be best to try the first 4 first, if it improves a lot, no need for the others, if not, go through all, something might be mucked further along the line.