Re: [Xen-devel] Benchmark Xen writes with sync - Xen ignores fsync, O_SYNC?

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 5 Apr 2012, Pablo Llopis wrote:
> Hi there Xen community,
> I am trying to benchmark and compare I/O in Xen/domU to native performance.
> 
> In order to do this I started trying to benchmark writes so as to avoid 
> caching effects that surely turn up when
> performing reads due to the page cache et al.
> However, I have quickly run into a problem: Xen domU reports that a 128MB 
> file is written at close to 300MB/s, while the
> disk's performance peaks at about 80MB/s (I observed this on a dom0 and on a 
> bare-metal kernel with no hypervisor).
> Please note that I fsync() after all writes in hopes to avoid the effect of 
> write buffers.ÂI have tried with O_SYNC as
> well, observing a similar outcome.
> I can confirm this writing a simple program, and verified exactly same 
> results running bonnie++ with the fsync() option
> turned on.
> 
> I am surprised to see writes reaching a throughput as high as 300MB/s, as the 
> disk surely isn't physically capable of
> reaching that bandwidth, meaning that writes are not being really synced to 
> disk.Â
> 
> Is this a bug in Xen, or is there a way to make Xen not ignore fsync, 
> fdatasync, O_SYNC, etc..?Â
> How would I proceed to measure and compare real read/write speeds on a Xen 
> domU ?
> 
> My disk drivers are specified with "file:/path/to/image.img,xvda,1,w" (I 
> could not get the tapdisk driver to work
> properly, I tried with vanilla 3.2 and 3.0.0 ubuntu kernels)
> Xen is version 4.1.1 and is running Oneiric domUs (kernel 3.0.0)
> For the dom0 I have a 3.2 vanilla kernel and a ubuntu (oneiric) 3.0.0 kernel

Are you using xend/xm to create the guest? In that case you are using
a loop device with blkback behind the scenes, that might go through the
disk cache.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel