This document discusses configuration limits and recommendations for KVM for IBM z Systems 1.1.2. It provides a table with recommended and maximum limits for categories like CPU, memory, networking, storage, and other resources. It then discusses each category in more detail, providing testing results and additional configuration tips.

1. KVM for IBM z Systems
KVM for IBM z Systems
Limits and Configuration Recommendations
This document can be found on the web, www.ibm.com/support/techdocs
Search for document number under the category of “White Papers.
Version Date: October 28, 2016
KVM for IBM z Systems Team

2. Special Notices
Please Note: Any performance data contained herein was determined in a controlled
environment. Therefore, the results obtained in other operating environments may vary
significantly. Some measurements may have been made on development-level systems
and there is no guarantee that these measurements will be the same on generally available
systems. Furthermore, some measurements may have been estimated through
extrapolation. Actual results may vary.
Trademarks
IBM, IBM logo, LinuxONE Emperor, LinuxONE Rockhopper, and z Systems are trademarks or registered trademarks of
the International Business Machines Corporation.
Adobe, the Adobe logo, PostScript, and the PostScript logo are either registered trademarks or trademarks of Adobe
Systems Incorporated in the United States, and/or other countries.
Cell Broadband Engine is a trademark of Sony Computer Entertainment, Inc. in the United States, other countries, or both
and is used under license therefrom.
InfiniBand and InfiniBand Trade Association are registered trademarks of the InfiniBand Trade Association.
Intel, Intel logo, Intel Inside, Intel Inside logo, Intel Centrino, Intel Centrino logo, Celeron, Intel Xeon, Intel SpeedStep,
Itanium, and Pentium are trademarks or registered trademarks of Intel Corporation or its subsidiaries in the United States
and other countries.
Java and all Java-based trademarks are trademarks of Sun Microsystems, Inc. in the United States, other countries, or
both.
Linux is a registered trademark of Linus Torvalds in the United States, other countries, or both.
Microsoft, Windows, Windows NT, and the Windows logo are trademarks of Microsoft Corporation in the United States,
other countries, or both.
OpenStack is a trademark of OpenStack LLC. The OpenStack trademark policy is available on the OpenStack website.
TEALEAF is a registered trademark of Tealeaf, an IBM Company.
UNIX is a registered trademark of The Open Group in the United States and other countries.
Worklight is a trademark or registered trademark of Worklight, an IBM Company.
All statements regarding IBM’s future direction and intent are subject to change or withdrawal without notice, and
represent goals and objectives only.
This document is current as of the initial date of publication and may be changed by IBM at any time. Not all offerings are
available in every country in which IBM operates.It is the user’s responsibility to evaluate and verify the operation of any
other products or programs with IBM products and programs.
THE INFORMATION IN THIS DOCUMENT IS PROVIDED “AS IS” WITHOUT ANY WARRANTY, EXPRESS OR
IMPLIED, INCLUDING WITHOUT ANY WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
PURPOSE AND ANY WARRANTY OR CONDITION OF NON-INFRINGEMENT. IBM products are warranted according
to the terms and conditions of the agreements under which they are provided.

3. © IBM Copyright, 2016 Version, October 28, 2016
Web location of document (www.ibm.com/support/techdocs) 3
KVM for IBM z Systems Limits and Configuration Recommendations
Table of Contents
Introduction..........................................................................................................................4
Limits Table.........................................................................................................................5
Discussion............................................................................................................................6

4. © IBM Copyright, 2016 Version, October 28, 2016
Web location of document (www.ibm.com/support/techdocs) 4
KVM for IBM z Systems Limits and Configuration Recommendations
Introduction
The following table summarizes recommended and maximum limits for KVM for IBM z
Systems 1.1.2. Following the table you will find additional information about each
category.
Terminology used in the table below.
Recommended Limit: A configuration that is inside defined limits that will be
covered by a support statement in terms of functional and performance issues.
Maximum Limit: A configuration that is inside defined limits that will be
covered by a support statement in terms of functional issues.
Note: a customer may define configurations outside these limits, however, such
configurations will not be supported.

5. © IBM Copyright, 2016 Version, October 28, 2016
Web location of document (www.ibm.com/support/techdocs) 5
KVM for IBM z Systems Limits and Configuration Recommendations
Limits Table
Category Function Recommended Limit Maximum Limit
CPU Overcommit 10:1 N/A
Host CPUs 101 (zEC12)
z13: 28-36 (1 drawer) 141 (z13)
Per Guest vCPUs Guest vCPUs <= Host CPUs 64
Memory Overcommit 2:1 N/A
Maximum Host 1 TB (zEC12, z13) 1 TB (zEC12) 8 TB (z13)
Networking OSA CHPIDs per Host 16 N/A
Virtual NICs per Host (with
OpenVswitch) 4096 8192
Virtual NICs per Guest 8 32
Storage Total attached zFCP LUNs 4096 64K
Total attached ECKD
Devices 4096 64K
Virtual Block Disks per
Host 4096 64K
Virtual Block Disks per
Guest 500 1024
HPM Guests per Service Class 512 1024
Service Classes per Host 512 1024
Other Guests per Host 512 4096

6. © IBM Copyright, 2016 Version, October 28, 2016
Web location of document (www.ibm.com/support/techdocs) 6
KVM for IBM z Systems Limits and Configuration Recommendations
Discussion
Each of the categories listed above are discussed in the paragraphs below.
CPU
The 10:1 vCPU:CPU overcommit ratio was tested with the J2EE Daytrader 3.0
application running on several KVM guests. All guests were running at an equivalent
and constant throughput. The effect on guest and overall host performance when
overcommitting resources varies greatly and can either show little effect or significant
effect. Careful selection of the combination of guest workloads that run on an
overcommitted host can help to ease the performance effects.
Memory
The 2:1 memory overcommit ratio was tested with the J2EE Daytrader 3.0 application
running on several KVM guests. All guests were running at an equivalent and constant
throughput. The effect on guest and overall host performance when overcommitting
resources varies greatly and can either show little effect or significant effect. Careful
selection of the combination of guest workloads that run on an overcommitted host can
help to ease the performance effects.
Networking
1. The network throughput between two LPARs is about half of the expected level when
both LPARs are using the same OSA device, when compared to the case where each
LPAR uses different OSA devices.
2. Due to a OSA capacity restriction, only 1000 Virtual Servers using MacVTap can be
started simultaneously per OSA card
3. The command ifconfig is obsolete. Please use
ip addr
ip link
ip –s link
4. In case the customer needs to modify the buffer count value for each network adapter,
without a reboot, the customer must use the following recommended procedure:
ip link set eth0 down && echo 0 > /sys/devices/qeth/0.0.f5f0/online && echo 64 >
/sys/devices/qeth0.0.f5f0/buffer_count && echo 1 > /sys/devices/qeth/0.0.f5f0/online && ip link set eth0
up
5. When a guest is configured to use an ethernet bridge (such as virbr0) using something
like the xml fragment below:

7. © IBM Copyright, 2016 Version, October 28, 2016
Web location of document (www.ibm.com/support/techdocs) 7
KVM for IBM z Systems Limits and Configuration Recommendations
<interface type=’bridge’>
<mac address=’00:06:fe:10:35:01’/>
<source bridge=’virbr0/>
<target dev=’net001’/>
<model type=’virtio’/>
<alias name=’net0’/>
<address type=’ccw’ cssid=’0xfe’ ssid=’0x0’
devno=’0x0002’/>
</interface>
The net001 device created by qemu has a default txqueuelen of 500. If the txqueuelen is
modified on the hypervisor to 2500 a significant increase in network performance occurs.
We suggest setting txqueuelen=2500 and buffercnt=128.
Storage
If you wish to access more than 511 scsi luns on a host, you need to add the following
kernel parameters to /etc/zipl.conf, and run zipl:
scsi_mod.max_luns=2000 scsi_mod.max_report_lunx=1999

8. © IBM Copyright, 2016 Version, October 28, 2016
Web location of document (www.ibm.com/support/techdocs) 8
KVM for IBM z Systems Limits and Configuration Recommendations
HPM
The total number of workloads could vary depending on the partition of service classes.
Other
The maximum number of concurrent live guest migrations – based on the default
configuration - is 32.