Re: [Qemu-devel] Why qemu write/rw speed is so low?

[Stefan Hajnoczi]

On Tue, Sep 13, 2011 at 10:38:28AM +0800, Zhi Yong Wu wrote:
> On Fri, Sep 9, 2011 at 6:38 PM, Stefan Hajnoczi
> <address@hidden> wrote:
> > On Fri, Sep 09, 2011 at 05:44:36PM +0800, Zhi Yong Wu wrote:
> >> Today, i did some basical I/O testing, and suddenly found that qemu write 
> >> and rw speed is so low now, my qemu binary is built on commit 
> >> 344eecf6995f4a0ad1d887cec922f6806f91a3f8.
> >>
> >> Do qemu have regression?
> >>
> >> The testing data is shown as below:
> >>
> >> 1.) write
> >>
> >> test: (g=0): rw=write, bs=512-512/512-512, ioengine=libaio, iodepth=1
> >
> > Please post your QEMU command-line.  If your -drive is using
> > cache=writethrough then small writes are slow because they require the
> > physical disk to write and then synchronize its write cache.  Typically
> > cache=none is a good setting to use for local disks.
> >
> > The block size of 512 bytes is too small.  Ext4 uses a 4 KB block size,
> > so I think a 512 byte write from the guest could cause a 4 KB
> > read-modify-write operation on the host filesystem.
> >
> > You can check this by running btrace(8) on the host during the
> > benchmark.  The blktrace output and the summary statistics will show
> > what I/O pattern the host is issuing.
>   8,2    0        1     0.000000000   337  A  WS 425081504 + 8 <-
> (253,1) 42611360

8 blocks = 8 * 512 bytes = 4 KB

So we are not performing 512 byte writes.  Some layer is changing the
I/O pattern.

Stefan