[libvirt-users] VM I/O performance drops dramatically during storage migration with drive-mirror

[Kashyap Chamarthy]

Cc the QEMU Block Layer mailing list (qemu-block at nongnu.org), who might
have more insights here; and wrap long lines.

On Mon, May 28, 2018 at 06:07:51PM +0800, Chunguang Li wrote:
> Hi, everyone.
> 
> Recently I am doing some tests on the VM storage+memory migration with
> KVM/QEMU/libvirt. I use the following migrate command through virsh:
> "virsh migrate --live --copy-storage-all --verbose vm1
> qemu+ssh://192.168.1.91/system tcp://192.168.1.91". I have checked the
> libvirt debug output, and make sure that the drive-mirror + NBD
> migration method is used.
> 
> Inside the VM, I use an I/O benchmark (Iometer) to generate an oltp
> workload. I record the I/O performance (IOPS) before/during/after
> migration. When the migration begins, the IOPS dropped by 30%-40%.
> This is reasonable, because the migration I/O competes with the
> workload I/O. However, during almost the last period of migration
> (which is 66s in my case), the IOPS dropped dramatically, from about
> 170 to less than 10. I also show the figure of this experiment in the
> attachment of this email.

[The attachment should arrive on the 'libvirt-users' list archives; but
it's not there yet --
https://www.redhat.com/archives/libvirt-users/2018-May/thread.html]

> I want to figure out what results in this period with very low IOPS.
> First, I added some printf()s in the QEMU code, and knew that, this
> period occurs just before the memory migration phase. (BTW, the memory
> migration is very fast, which is just about 5s.) So I think this
> period should be the last phase of the "drive-mirror" process of QEMU.
> So then I tried to read the code of "drive-mirror" in QEMU, but failed
> to understand it very well.
> 
> Does anybody know what may lead to this period with very low IOPS?
> Thank you very much. 
> 
> Some details of this experiment: The VM disk image file is 30GB
> (format = raw,cache=none,aio=native), and Iometer operates on an 10GB
> file inside the VM. The oltp workload consists of 33% writes and 67%
> reads (8KB request size, all random). The VM memory size is 4GB, most
> of which should be zero pages, so the memory migration is very fast.
>
> --
> Chunguang Li, Ph.D. Candidate
> Wuhan National Laboratory for Optoelectronics (WNLO)
> Huazhong University of Science & Technology (HUST)
> Wuhan, Hubei Prov., China


-- 
/kashyap