Re: [Qemu-devel] Greate difference of disk I/O performance for guest on Qemu-2.30 of CentOS.

[Stefan Hajnoczi]

On Fri, Jun 19, 2015 at 08:40:11AM +0800, cauchy-love wrote:
> 
> the iozone  command line is:
> ./iozone -i 0 -i 1 -f ./iotmp -Rab ./iotmp.xls -g 8G -n 4G -c
> 
> The problem is why the performance difference is so big for different linux 
> kernels. The guest's io performance test is by both FTP tools and raw write, 
> which demonstrate the much lower performance of  Centos 7 (1/10 of that on 
> Centos 6.5). And how can I debug this problem?

You are missing the -I command-line option for O_DIRECT.

When you run without -I you are not benchmarking just disk I/O but also
page cache performance, which varies depending on the amount of RAM
available.  This means you cannot compare guest results with host
results.

Stefan

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