On October 12, 2011, IBM announced
z/VM V6.2 - Accelerate the Journey to Smarter Computing:
the newest release introducing Single System Image and Live Guest
Relocation, a first for the mainframe.
z/VM V6.2 will be orderable on November 29, 2011.
The z/VM V6.2 release introduces the Single System Image feature
(VMSSI), which is a separately orderable priced feature of z/VM V6.2.
The VMSSI Feature enhances the z/VM systems
management, communications, disk management, device mapping, virtual
machine definition management, installation, and service functions to
enable multiple z/VM systems to share and coordinate resources within
a Single System Image (SSI) structure. This combination of enhanced
functions provides the foundation that enables
Live Guest Relocations (LGR), which is the ability for a Linux
guest to be moved from one z/VM system to another within the SSI
Planning for a Single System Image and Live Guest Relocations
There are several reasons why you might need to relocate a virtual
server while keeping the server available.
You might need to relocate a virtual server for
workload rebalancing, or to do software maintenance or hardware
Before you relocate a guest, there are architectural, disk, memory, and
networking requirements plus guidelines you must understand.
Below are some hints to help with installation of the VMSSI feature
and tips to get you started relocating a Linux guest.
Hints and Tips:
- VMSSI Installation
Please keep in mind even if you have previous experience with
installation and service of z/VM, it is important that you read
the instructions for installation of z/VM 6.2 with or without
the VMSSI feature.
To plan and prepare for z/VM V6.2 in advance of the general
we encourage you to familarize yourself by
referring to the following publications: z/VM: Getting
Started with Linux on System z and the z/VM:
CP Planning and Administration, chapter 25.
- ISFC Set-up
An SSI cluster must have direct logical links between all
All SSI clusters use ISFC for intra-cluster communication and live guest
relocation. ISFC uses CTC devices. For maximum throughput, when you are
setting up your network, follow the Guidelines for
planning your network in an SSI cluster located in the
z/VM: Getting Started with Linux on System z, chapter 2.
Other things being equal, faster CTC speeds increase throughput
and result in shorter relocations.
Factors that can affect relocation are:
- Virtual Machine Memory
The size and use of the virtual machine's memory can
affect relocation performance. Parts of the processing for
relocation are proportional to the size of the virtual machine.
The cost of this processing
increases with larger virtual machines.
Relocation performance is also impacted by the frequency
and amount of memory being changed in the virtual machine.
- Matching Virtual Machine Configurations
To prepare for live guest relocation, you need to ensure that the
virtual machine has
a relocatable configuration and that a matching configuration can be set
up on the
destination system. There are configuration attributes the guest must
because they cannot be relocated, and there are also characteristics the
system must have in order to provide an identical virtual machine
configuration. For information on configuration requirements
and on verifying a virtual machine's eligibility to relocate,
please refer to the z/VM: CP Planning and
Administration, chapter 27.
- CPU Utilization
The z/VM V6.2 Single System Image will synchronize all the members in the
cluster. You must ensure that you have allotted enough system resources
to account for the necessary synchronization and communication among
members. Understand that independent systems not formally clustered do
not require this synchronization overhead. After initialization, the
synchronization overhead is relatively low. Communication between
members does increase during negotiations for access to devices and
other resources, as well as during Live Guest Relocation.
For example two independent
systems that today run fine at peak utilization (close to 100%), when
joined in a cluster may have performance problems.
For z/VM members that are running as
a second level z/VM system, they should not be waiting for CPU more
than 10% of the time. For additional details refer to the Resource
Limit Conditions section of the
z/VM: CP Planning and Administration, chapter 27.
- Paging and Other System Resouces
To prepare for Live Guest Relocation, the target system must have enough
system resource during and after the relocation. You will need to ensure
your paging space is adequate. To be safe, there should be twice as much
space available as the total virtual memory that can be defined on the
The easiest way to check on this aspect of system resources is to issue
CP QUERY ALLOC PAGE command
which will show the percent used, the slots available,
and the slots in use.
If you add in the size of the virtual machine(s) being
relocated (a 4KB page = a 4KB slot) to the slots in use, and that brings
the in use percentage over 50%, that may have an undesirable impact on
system performance. Remember this query command provides only a snap
shot in time.
- Real Memory
Real memory resources are important for both the source and the
destination systems for relocations. You will need enough
real memory 1) to hold buffers during the relocation on both systems,
and 2) to accommodate the incoming guest's
working set afterward on the target system.
Relocation performance will also be affected by the level of overall
resource constraint for both the source and destination systems.
- Linux Distributions and Live Guest Relocations
With the introduction of Live Guest Relocation among members of your SSI,
it is increasingly important to identify the level of Linux on System z
that is running within each member. Although z/VM has performed
considerable testing, each customer environment is unique with different
hardware, software and networking levels. Therefore not all
combinations may have been identified. The following PDF is being
provided as a guideline to identify if the level of Linux on System z
was included within the z/VM 6.2 relocation test environments.
LGR Linux Distribution Information
Please keep in mind that all systems are unique,
consisting of different hardware/software levels, Linux levels,
networks and configurations.
The recommendation and best practice will be to verify a relocation
between members by testing it prior to a planned relocation.
For more information, see Chapter 27. Preparing for Guest Relocations in
a z/VM SSI Cluster in the z/VM: CP Planning and Administration. For more
information about networking devices, see Chapter 6. Live Guest
Relocation Networking Considerations in z/VM: Connectivity. Another
source of information can be found at
virtual networking hints and tips.