Hello everybody.
Today I was migrating VMs with different levels of memory (using libvirt) and found something strange concerning memory ballooning.
I took three measurements:
(A) mem: 16GiB, maxmem: 16GiB, migration-time: 7.3sec
(B) mem: 8GiB, maxmem: 8GiB, migration-time: 5.2sec
(C) mem: 8GiB, maxmem: 16GiB, migration-time: 7.3sec
As you can see I used memory ballooning at (C), but this did not decrease the duration of migration.
I expected the duration to decrease due to less memory to be copied.
The migration was cold and the VMs are of KVM type.
This was the migration call:
virsh migrate <vm-name> qemu+ssh://<dest-host>/system
I confirmed that the VM had adapted the memory balloon by calling "free" in the VM via ssh.
The "total" column stated some less than the previously set "mem" value.
Does anybody knows why ballooning does not cause the migration to perform faster?
Is it maybe a bug?
If further information are required or if this should better be posted on KVM or libvirt mailing lists, please let me know.