By: Paul Ward
This article describes two new network configuration options that were recently made available in PowerVC v1.2.3. The first of these new options is the ability to specify MTU (maximum transmission unit) on network templates. The second is a new cloud-init module that allows you to utilize your DNS servers to set your virtual machines’ hostname upon first boot after deployment.
Note: These new network configuration options are only available when using cloud-init which became an option in PowerVC v1.2.3. For more information on using cloud-init, see:
MTU (maximum transmission unit) is the maximum size of data packets a network device is configured to handle. This applies to switches, network interface cards, etc. The standard MTU used across most of the internet is 1500 bytes. However, this is a configurable property that can be used to tune network performance to one‘s particular needs.
A common use case for using a non-standard MTU is on the private network of a large data center. In this environment, the network traffic is mainly consumed by the transmission of large quantities of data. Configuring the switches within the data center to use an MTU of 9000 bytes can greatly decrease the packet overhead, since there will be fewer packet headers compared to 1500 byte packets, and increase throughput dramatically.
However, configuring your network infrastructure to allow larger packet sizes is only half the battle. The servers connected to the network must have their interfaces configured with this larger MTU size as well, otherwise they will still be sending 1500 byte packets.
For a real world example, you may have a virtual machine that has one interface on your public network and a second interface on your private network in order to access a database server:
In release 1.2.3 of PowerVC, we have added an option in the user interface where an administrator can specify the MTU size on a given network:
Whenever a VM is deployed using this network, the network interface receiving this network’s configuration will have it’s MTU set to the value specified using the cloud-init activation engine.
NOTE: Setting the MTU size to a non-standard value should only be done by network administrators that fully understand all the implications. Setting the MTU to a value smaller than what is configured on your network switches should be fine, but setting it to a value higher can cause severe network performance issues as it can result in the packets needing to be fragmented before routing or, worse yet, they get dropped entirely. If you are not familiar with your network infrastructure, the best thing to do is leave MTU blank when creating the network in PowerVC. This will cause activation to skip setting MTU and default to the OS-specific default behavior.
Set Hostname from DNS
Another pain point many clients have voiced to us is the discrepancy between the hostname configured on a deployed VM vs the hostname configured in their DNS server for that VM’s IP address.
Therefore, in PowerVC 1.2.3, we’ve provided a new cloud-init module, named set_hostname_from_dns, which will allow a virtual machine to get its hostname by doing a reverse DNS lookup against its IP address. This gives the user more flexibility and control to use their existing DNS server to hand out hostnames rather than having to specify hostname on the deploy. This can be especially helpful in easing the automation of multi-VM deployments, automatically allowing each one to get the proper hostname from DNS.
This new module will work with both IPv4 and IPv6 addresses. If the address is not resolvable, behavior reverts back to default OpenStack behavior and sets the hostname to the virtual machine name specified on the deploy.
Detailed instructions on using this module can be found at http://www-01.ibm.com/support/knowledgecenter/SSXK2N_1.2.3/com.ibm.powervc.standard.help.doc/powervc_install_cloudinit_hmc.html. It is present in the cloud-init rpm packaged with PowerVC 1.2.3 for linux OSes and version 0.7.5 and later of cloud-init for AIX.
The above enhancements give the user a lot more flexibility with their network configuration, but there are certainly more ways we could make life easier for you. Please let us know, we love to hear from our users and how we could make their systems administration easier!