** Changed in: qemu
Status: Incomplete => New
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https://bugs.launchpad.net/bugs/498417
Title:
cache=writeback on disk image doesn't do write-back
Status in QEMU:
New
Bug description:
I noticed that qemu seems to have poor disk performance. Here's a
test that has miserable performance but which should be really fast:
- Configure qemu to use the disk image with cache=writeback
- Configure a 2GiB Linux VM on an 8GiB Linux host
- In the VM, write a 4GiB file (dd if=/dev/zero of=/tmp/x bs=4K count=1M)
- In the VM, read it back (dd if=/tmp/x of=/dev/null bs=4K count=1M)
With writeback, the whole file should end up in the host pagecache.
So when I read it back, there should be no I/O to the real disk, and
it should be really fast. Instead, I see disk activity through the
duration of the test, and the performance is roughly the native hard
disk throughput (somewhat slower).
I'm using version 0.11.1, and this is my command line:
qemu-system-x86_64 -drive
cache=writeback,index=0,media=disk,file=ubuntu.img -k en-us -m 2048
-smp 2 -vnc :3100 -usbdevice tablet -enable-kvm &
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