[Qemu-devel] [Bug 498417] Re: cache=writeback on disk image doesn't do write-back

[Thomas Huth]

QEMU 0.11 / 0.12 is pretty much outdated nowadays ... can you still
reproduce this issue with the latest version of QEMU?

** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/498417

Title:
  cache=writeback on disk image doesn't do write-back

Status in QEMU:
  Incomplete

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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