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

[Chunguang Li]

> -----Original Messages-----
> From: "Kashyap Chamarthy" <kchamart at redhat.com>
> Sent Time: 2018-05-28 20:05:05 (Monday)
> To: "Chunguang Li" <lichunguang at hust.edu.cn>
> Cc: libvirt-users at redhat.com, qemu-block at redhat.com, dgilbert at redhat.com
> Subject: Re: [libvirt-users] VM I/O performance drops dramatically during storage migration with drive-mirror
> 
> Cc the QEMU Block Layer mailing list (qemu-block at nongnu.org), who might
> have more insights here; and wrap long lines.
> 

Hi, Kashyap, thank you very much.

> 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 don't know whether the attachment could arrive on the list archives. 
So I make it available through this link:
https://pan.baidu.com/s/1pByKQtJ7VdFCDbX-ZMyOwQ

Chunguang
> > 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