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

Cc the QEMU Block Layer mailing list (qemu-block(a)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