Jumbo frames
A larger MTU means fewer packets for the same data. The gain is real and the failure mode is confusing, so the rule is simple: private networks only.
Where it helps
Bulk transfer between your own machines: storage replication, backups, database replication, a storage back end. Fewer, larger packets mean fewer interrupts and less per-packet overhead, and on a 10 Gbit link doing sustained transfer the difference is measurable.
Where it must not be used
On any interface facing the public internet. The internet's MTU is 1500, and a host emitting 9000-byte frames onto a path that cannot carry them produces exactly the symptom in MTU, fragmentation and tunnels: ping works, small requests work, large transfers hang forever.
Jumbo frames belong on a dedicated private interface or a VLAN, never on your public one.
Everything on the path must agree
Both servers, both switch ports, and any router between. One device at 1500 in the middle silently drops the large frames, and because it only affects big packets the symptom looks like an application bug.
That is why this has to be agreed with us first: the switch side is ours. Open a ticket saying which machines and which network, and we will set it.
Configuring it
To test:
ip link set dev eno2 mtu 9000
Permanently, in netplan: mtu: 9000 under that interface. On a bond or a
VLAN, set it on the underlying interface as well as the logical one, or the lower
layer caps you.
Test it honestly
Send an unfragmented packet of exactly the payload size that fits:
ping -M do -s 8972 -c 3 10.0.0.6
8972 plus 28 bytes of headers is 9000. If that succeeds, the whole path carries
jumbo frames. If it fails while -s 1472 succeeds, something on the path
is still at 1500 — and now you know before you have built anything on top of
it.
Measure whether it was worth it
iperf3 -c 10.0.0.6 -P 4 -t 30 before and after. On a clean 10 Gbit link
with modern cards and offloads already doing segmentation, the gain is often smaller
than people expect — GRO and TSO already batch much of the work in hardware.
See network cards and offloads.
If the numbers do not move, set it back. A setting that complicates your network without improving it is a liability, not a tune.
The usual arrangement
Two interfaces: the public one at 1500, a private one at 9000 for replication and backups. Clean, no interaction between them, and the failure mode cannot reach your customers. See connecting your own servers.
Still stuck? Mail support@novogara.com — an engineer answers, at any hour. Back to the knowledge base