[Qemu-discuss] Question on i/o performance for image-file vs partition to emulate disk in KVM-QEMU guest

[sanjay]

Hi! QEMU allows host to pass in (i) qcow2/raw image file or (ii) host disk partition to be emulated as disks within the KVM-QEMU guest. When a image file is passed to QEMU for emulation, I was assuming that the that i/o throughput will be lower as it needs to go through the Host File System (GuestFS->QEMU Layer->HostFS->Driver).

However, during a test found that the guest i/o throughput for both the options appears to be close, in fact the in several cases option (i) appears to have a better throughput. Is my assumption incorrect ? Any help will be appreciated.

Context: (cache disabled on guest, both used virtio driver, partition passed in is an unformatted logical partition in the host with SATA disk, stats from iozone).

[i] -drive file=/temp/test_pool1/vol1.img,if=none,id=drive-virtio-disk1,format=raw -device virtio-blk-pci,bus=pci.0,addr=0x6,drive=drive-virtio-disk1,id=virtio-disk1

[ii] -drive file=/dev/sdc7,if=none,id=drive-virtio-disk1, format=raw -device virtio-blk-pci,bus=pci.0,addr=0x6,drive=drive-virtio-disk1,id=virtio-disk1

 

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Regards,

Sanjay