What is the unit of "migration_max_bandwidth" in vdsm.conf ?

Solution Verified - Updated

Environment

  • Red Hat Enterprise Virtualization 3.x
  • Red Hat Virtualization 4.x

Issue

  • To increase the migration_max_bandwidth value ( to allow large VMs to migrate within the timeout period)
  • What unit does migration_max_bandwidth expect?
  • Is that parameter applied per VM migration ?

Resolution

Note: in RHV 4, migration policy should be configured via the engine, and vdsm.conf manual config would be overwritten by engine values.
Units of migration_max_bandwidth option are MiBps, that is megabytes per second.
Please read this article before applying any changes to your environment:
Understanding live migration "migration_max_bandwidth" and "max_outgoing_connections" parameters in vdsm.conf

Note: the below changes have to be applied to all the hosts in the environment / all the hosts within the same cluster.

1. Put the host in maintenance mode.

2. Make the following changes in the vdsm.conf file.

The exact config are. 

[root@example.com]#cat /etc/vdsm/vdsm.conf
[vars]
trust_store_path = /etc/pki/vdsm
ssl=true

# Maximum bandwidth for migration, in MiBps, 0 means libvirt's
# default, since 0.10.x default in libvirt is unlimited
# migration_max_bandwidth = 32
migration_max_bandwidth = 100

[addresses]
management_port=54321

Note: You can set the migration_max_bandwidth = 1024, as per requirement. However, you also consider capability of network which is used for migration.

3. Restart the vdsmd service.

# service vdsmd restart

Diagnostic Steps

1. The unit is "MiBps" as mentioned in /usr/share/doc/vdsm-*/vdsm.conf.sample:

# Maximum bandwidth for migration, in MiBps, 0 means libvirt's
# default, since 0.10.x default in libvirt is unlimited
# migration_max_bandwidth = 32

2. It is per vm :

        if MigrationMonitorThread._MIGRATION_MONITOR_INTERVAL:
            monitorThread = MigrationMonitorThread(self._vm)
            monitorThread.start()

        try:
            maxBandwidth = config.getint('vars', 'migration_max_bandwidth')
            self._vm._dom.migrateToURI(duri, libvirt.VIR_MIGRATE_LIVE |
                                    libvirt.VIR_MIGRATE_PEER2PEER, None, maxBandwidth

Note, in older version , there are following "confusing" comment may include in /etc/vdsm/vdsm.conf file or vdsm.conf.sample file. 

# Maximum bandwidth for migration, in mbps, 0 means libvirt's default (30mbps?)

usually mbps indicate Mega bits per second. However the units is MiBps actually.

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