KVM for IBM z Systems Limits and Configuration Recommendations

1. KVM for IBM z Systems
KVM for IBM z Systems
Limits and Configuration Recommendations
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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.
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KVM for IBM z Systems Limits and Configuration Recommendations
Table of Contents
Introduction..........................................................................................................................4
Limits Table.........................................................................................................................5
Discussion............................................................................................................................6

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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.

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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

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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:

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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

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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.