>Did you prefill the image? Amplification could come from cluster allocation. Yes! I forgot to mention that I created the qcow2 image with 'preallocation=metadata', and I have allocated the data blocks with dd in VM.
Creating image in host: qemu-img create -f qcow2 -preallocation=metadata test.qcow2 100G Allocating the blocks in VM: dd if=/dev/zero of=/dev/vdb bs=1M where vdb is the target image. At 2014-06-23 11:01:20, "Fam Zheng" <f...@redhat.com> wrote: >On Mon, 06/23 10:06, lihuiba wrote: >> Hi, all >> >> >> I'm using a qcow2 image stored on a SSD RAID1 (2 x intel S3500), and I'm >> benchmarking the >> system using fio. Although the throughput in VM (with KVM and virtio >> enabled) is acceptable (67% >> of thoughtput in host), the IOPS performance seems is extremely low ---- >> only 2% of IOPS in host. >> >> >> I was initially using qemu-1.1.2, and I also tried qemu-1.7.1 for >> comparison. There was no significant >> difference. >> >> >> In contrast, raw image and LVM perform very well. They usually achieve 90%+ >> of throughput and >> 60%+ of IOPS. So the problem must lie in the QCOW2 image format. >> >> >> And I observed that, when I perform 4KB IOPS benchmark in VM with a QCOW2 >> image, fio in VM reports >> it is reading 9.x MB/s, while iostat in host reports the SSD is being read >> 150+ MB/s. So QEMU or QCOW2 >> must have amplified the amount of read by nearly 16 times. >> >> >> So, how can I fix or tune the performance issue of qcow2? > >Did you prefill the image? Amplification could come from cluster allocation. > >Fam > >> >> >> Thanks! >> >> >> >> >> PS: >> 1. qemu parameters: >> -enable-kvm -cpu qemu64 -rtc base=utc,clock=host,driftfix=none -usb -device >> usb-tablet -nodefaults -nodefconfig -no-kvm-pit-reinjection -global >> kvm-pit.lost_tick_policy=discard -machine pc,accel=kvm -vga std -k en-us >> -smp 8 -m 4096 -boot order=cdn -vnc :1 -drive >> file=$1,if=none,id=drive_0,cache=none,aio=native -device >> virtio-blk-pci,drive=drive_0,bus=pci.0,addr=0x5 -drive >> file=$2,if=none,id=drive_2,cache=none,aio=native -device >> virtio-blk-pci,drive=drive_2,bus=pci.0,addr=0x7 >> >> >> 2. fio parameters for IOPS: >> fio --filename=/dev/vdb --direct=1 --ioengine=libaio --iodepth 32 --thread >> --numjobs=1 --rw=randread --bs=4k --size=100% --runtime=60s >> --group_reporting --name=test >> >> >> 3. fio parameters for throughput: >> fio --filename=/dev/vdb--direct=1 --ioengine=psync --thread --numjobs=3 >> --rw=randread --bs=1024k --size=100% --runtime=60s --name=randread >> --group_reporting -name=test >> >> >> >
