Fairly straight forward this time, you’ve configured your
MTU/jumbo frames to be
9000 on your client and destination devices (say a laptop/desktop/server/san/nas) and on ALL your switching devices in between – you’ve done that right? ;)
So the next step is, we want to test if our new
9000 byte MTU is actually working and we can reap the benefits of a larger packet size (whether it’s on iSCSI, LAN, whatever) being of course a higher latency but also higher throughput. This depends on the OS you are running – on Mac OSX (that I run) it’s:
ping -D -s 8184 [destinationIP]
On Linux it’s:
ping -M do -s 8972 [destinationIP]
On Windows it’s:
ping -f -l 9000 [destinationIP]
The reason for the
8972 on *nix devices is that the ICMP/ping implementation doesn’t encapsulate the
28 byte ICMP (8) + IP (20) (ping + standard internet protocol packet) header – thus we must take the
9000 and subtract
28 = 8972.
On mac’s even though they are *nix kernels, the ping implementation only supports packets
8192 in size so we must remove the ICMP (
8 byte) header as the ping implementation has already included the
20 byte IP header,
8192 - 8 = 8184.
(Apple macs DO support packets up to 9000 bytes, just the ICMP implementation they sport doesn’t…)
EDIT 31/10/13: According to BernieC in a comment here OSX does support 9000+ byte packets if you run the following command to increase its maximum datagram size:
sudo sysctl -w net.inet.raw.maxdgram=16384
It is also important to understand where I got my values from this is an IP packet’s layout, you can see the IP info is 20 bytes:
If you’ve forgotten to enable
jumbo frames/9k MTU on your client device you’re sending the ping from you’ll see:
PING xxx.xxx.xxx.xxx (xxx.xxx.xxx.xxx): 8184 data bytes ping: sendto: Message too long
If you have enabled jumbo frames on your client but not the destination (or a switch in between) you’ll see:
PING xxx.xxx.xxx.xxx (xxx.xxx.xxx.xxx): 8184 data bytes Request timeout for icmp_seq 0
If you’ve done everything right and you’re set up ready to go you’ll get this:
PING xxx.xxx.xxx.xxx (xxx.xxx.xxx.xxx): 8184 data bytes 8192 bytes from xxx.xxx.xxx.xxx: icmp_seq=0 ttl=128 time=0.714 ms
Now rejoice in your lovely jumbo-framey goodness and a good 20-30% in sustained data throughput.
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