NFS Performance Tuning for HP-UX 11.0 and 11i Systems

nfs performance tuning for hp-ux 11.0 and 11i systems page 100
Notes:
Page 100July 22, 2002
Copyright 2002 Hewlett- Packard Company
nfs/udp
vs.
nfs/tcp
Network Switch Buffering Issues
Customer Reported Problem
• High numbers of NFS/UDP retransmissions and timeouts
• UDP packets were being dropped by the network switch
• The same switch was NOT discarding TCP packets
Results of Investigation
The network hardware vendor confirmed that they dedicate
75% of the buffer memory in their switch for TCP/IP traffic
and only 25% for UDP traffic. This gives NFS/TCP an
advantage, albeit hardware-induced.
In some of HP’s largest NFS customer installations, we’ve seen cases where high
numbers of NFS/UDP retransmissions and timeouts were occurring, resulting in
relatively poor read and write performance.
After much investigation, it was determined that UDP packets were being discarded
by the network switch used by the customer. When the switch vendor investigated
the log files on the switch they discovered that the UDP buffers in the switch were
overflowing which led to the dropped packets. When asked why the switch was
not also dropping TCP packets the vendor explained that they design their switches
with 75% of the buffer memory dedicated for TCP/IP traffic and only 25% for UDP.
The switch therefore had sufficient memory to keep up with the TCP traffic but not
enough to handle the UDP load.
It is likely that many network hardware vendors configure their switch buffer
memory in this manner, so this problem is not confined to large-scale
implementations using a specific vendor’s switch. In these environments NFS/TCP
has an advantage over UDP, albeit hardware-induced.