Re: [Qemu-devel] extremely low IOPS performance of QCOW2 image format on an SSD RAID1

[Fam Zheng]

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
> 
> 
>