Re: [Xen-users] GPLPV Disk performance block device vs. file based backend

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Thu, Sep 20, 2012 at 05:16:53PM +0700, Fajar A. Nugraha wrote:
> On Thu, Sep 20, 2012 at 3:57 PM, Dion Kant <dion@xxxxxxxxxx> wrote:
> 
> > name="wsrv-file"
> > disk=[ 'file:/var/lib/xen/images/wsrv-file/disk0.raw,xvda,w', ]
> 
> 
> > name="wsrv-bd"
> > disk=[ 'phy:/dev/vg0/wsrv-bd,xvda,w', ]
> 
> > Now I measure more than a factor 3 better I/O performance on the file
> > based VM as compared to the block device based VM. I don't think it is a
> > cache issue which is tricking me.
> 
> I'm 99.9% sure it tricks you :)
> 
> > sync; time (dd if=/dev/zero of=test.bin bs=4096 count=5000000; sync)
> 
> dd is terrible for benchmark purposes. I'd suggest fio, random rw,
> data size at least twice RAM.
>

you can tell dd to do direct io bypassing caches so you get proper results:
iflag=direct or oflag=direct

The biggest limitation is that dd is always single threaded (unless you launch 
multiple copies).

-- Pasi


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