IBM Storwize V7000 Unified Problem Determination Manual page 405

Jumbo frames are Ethernet frames with a size in excess of 1500 bytes. The
maximum transmission unit (MTU) parameter is used to measure the size of
jumbo frames.
The Storwize V7000 Unified supports 9000-bytes MTU. Refer to the CLI
command cfgportip to enable jumbo frame. This command is disruptive as the
link flips and the I/O operation through that port pauses.
The network must support jumbo frames end-to-end for this to be effective;
verify this by sending a ping packet to be delivered without fragmentation. For
example:
v Windows:
ping -t <iscsi target ip> -S <iscsi initiator ip> -f -l <new mtu size - packet overhead (usually 36, might differ)>
The following command is an example of a command that is used to check
whether a 9000-bytes MTU is set correctly on a Windows 7 system:
ping -t -S 192.168.1.117 192.168.1.217 -f -l 8964
The following output is an example of a successful reply:
192.168.1.217: bytes=8964 time=1ms TTL=52
v Linux:
ping -l <source iscsi initatior ip> -s <new mtu size> -M do <iscsi target ip>
v ESXi:
ping <iscsi target ip> -I <source iscsi initiator ip> -s <new mtu size - 28> -d
3. Verify the switch's port statistic where initiator/target ports are connected to
make sure that packet drops are not high.
Review network architecture to avoid any bottlenecks and oversubscription.
The network needs to be balanced to avoid any packet drop; packet drop
significantly reduces storage performance. Involve networking support to fix
any such issues.
4. Optimize and utilize all iSCSI ports.
To optimize Storwize V7000 Unified resource utilization, all iSCSI ports must be
used.
v Each port is assigned to one CPU, and by balancing the login, one can
maximize CPU utilization and achieve better performance. Ideally, configure
subnets equal to the number of iSCSI ports on the Storwize V7000 Unified
node. Configure each port of a node with an IP on a different subnet and
keep it the same for other nodes. The following example displays an ideal
configuration:
Node 1
Port 1: 192.168.1.11
Port 2: 192.168.2.21
Port 3: 192.168.3.31
Node 2:
Port 1: 192.168.1.12
Port 2: 192.168.2.22
Port 3: 192.168.3.33
v Avoid situations where 50 hosts are logged in to port 1 and only five hosts
are logged in to port 2.
v Use proper subnetting to achieve a balance between the number of sessions
and redundancy.
5. Troubleshoot problems with PFC settings.
Chapter 5. Control enclosure
377