Kvm for ibm_z_systems_v1.1.2_limits

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

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.

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© 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


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.


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


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:


<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


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.