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

[Stefan Hajnoczi]

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.

I suggest changing your fio block size to 8 KB if you want to try a
small block size.  If you want a large block size, try 64 KB or 128 KB.

Stefan