On Mon, May 28, 2018 at 06:17:10PM +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. > > > > > 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.
IOPS dropped from 170 to less than 10. That could be because QEMU or the storage device is slow at completing requests due to the other activity (drive-mirror). But it could also be because the guest simply isn't submitting many I/O requests! So I think the first step is to determine how much I/O the guest is submitting. There are several ways of doing this. You can enable the virtio_blk_handle_write and virtio_blk_handle_read trace events if you are using virtio-blk (see docs/devel/tracing.txt). Or you could use iostat(1) inside the guest to observe the number of completed requests + queue size. If the guest is not submitting the expected number of requests then you'll need to investigate why Iometer is starved. > 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 >
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