I have a Windows Server VM which connects to my Storage VMs running CentOS in another Data Centre over SFTP and FTP.
The uplink between the servers are all 10Gbps and more and both VMs are ESXi machines connected directory to the uplink and configured with valid static IP addresses (different hosts ofc since they are in different Data Centres).
When I try to upload files (large files, usually above 500MB) from the Windows machine to the CentOS machine over FTP or SFTP the transfer speed is around 6~40MB/s which is fine for the time being but after a few days of work the transfer speed goes down to 300~500Kbps.
The strange thing is that restarting the Network service on the CentOS box resolves the issue for a few days untill it happens again...
I have around 10 cloned pairs of the machines described above and they all have the same issue.
EDIT 1: There are no limitations on the network or the links applied by the Data Centre that we know of. We can make a request for them to look into the issue with enough evidence if enough symptoms point in that direction.
EDIT 2: Both machines have free CPU, Memory and IOPS.
SOLUTION / CAUSE: The issue was caused by network limitations from the Data Centre network on the Destination side.