[libvirt] virDomainMigrate, "suitable default" for omitted bandwidth parameter

[Daniel P. Berrange]

On Tue, Feb 15, 2011 at 06:49:38PM +0100, Thomas Treutner wrote:
> Hi,
> 
> does somebody know what the following paragraph exactly means resp.
> what it should mean?
> 
> "The maximum bandwidth (in Mbps) that will be used to do migration
> can be specified with the bandwidth parameter. *If set to 0, libvirt
> will choose a suitable default*."

In this case we choose to leave the default on whatever the QEMU
default is.

> http://libvirt.org/html/libvirt-libvirt.html#virDomainMigrate
> 
> What is the "suitable default"? I looked through the code for qemu
> and the only call to qemuMonitorSetMigrationSpeed() I can find is in
> ./src/qemu/qemu_driver.c:8406, using libvirt 0.8.7. When I remember
> correctly the second condition in a conjunction will not be
> evaluated if the first one evaluates to false? So if resource == 0,
> no limit will be set?
> 
> I ask because I discovered that qemu is live migrating with a hard
> coded throttle of 32MiB/s for historic reasons, which is an
> activated handbrake if you have GBit Ethernet and additionally
> annoying when thinking about qemu's broken way of live migration (no
> maximum amount of iterations, no forced action, no error message, no
> abortion - no *nothing*.). Effectively *using* GBit Ethernet often
> solves this problem as the bandwidth to transfer dirty pages is
> quadrupled.
> 
> Also see qemu mailing list, Message-ID: <4D52D95D.3030300 at scripty.at>

Yep, QEMU needs to remove that limit IMHO

> 
> There was a short discussion on IRC where concerns of "breaking
> libvirt" when deactivating the default limit were stated. If there
> really are applications that depend on handbraked live migration, I
> think these applications just should pass the limit they need to
> virDomainMigrate(). What do you think?

Yes, they should not be relying on any default, since the default
is unspecified.

Daniel
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