FTP error when doing file transfer

Finally resolved by setting NIC on FTP server from 'half-duplex' to 'full duplex'.

Before zeroing in on the FTP aspect, you should perform some synthetic tests to rule out other causes. (I've assumed both the client and the server are Linux since you are running vsftpd.)

• Network packet drops: Check the output of cat /proc/net/dev on the client and server to see if there are any errors or dropped packets.
• Network load testing: Use a throughput testing tool such as iperf with a long test time to see if you can reproduce the issue. Monitor throughput with cat /proc/net/dev.
• MTU: If you're using 10GbE, you may have jumbo frames enabled (that is, MTU size greater than 1500). If you do, make sure that all the hosts and network devices support jumbo frames and that you aren't filtering ICMP packets anywhere (ICMP packets are used for MTU path discovery).
• Server disk: On the server, use dd to read from /dev/zero and write to a local file on the target filesystem. Monitor throughput with iostat.
• Client disk: May not need to test this given that you can FTP successfully to other servers from this client.

If any of the above tests show problems, you should have a clue about where to look next. Otherwise, look into the FTP side:

• If a transfer has slowed down, what happens if you disconnect and then resume the transfer? Is it slow straight away, or does it go fast again before slowing down?
• Do you get different behaviour when initiating the transfer from the client side vs the server side (i.e. connecting to an FTP server on the client)?
• Check the memory usage of the FTP server and client using top. If there's a memory leak something may be swapping and slowing everything down.
• Check the CPU usage of the FTP server and client for any strange behaviour.