Re: [Xen-devel] Does Xen hypervisor overwrite O_DIRECT setting of Linux2.6 kernel?

["Liang Yang" <liang_yang@xxxxxxxxxxx>]

Hi Ian,

Thanks a lot for your prompt reply.

I'm doing physcial drive performance testing for SAS drives. All the SAS 
drives under test are indeed raw drives (no filesystems on it). Each SAS 
drive is assigned maximum 32 outstanding I/Os. Linux, Xen and benchmark tool 
IOMeter are installed on a separate SATA drive. I don't know how disk zoning 
could be related to this issue.
Only for sequential write of small packets (512B and 1KB), Xen domain0 
outperform Linux native. For sequential/random read (512B, 1K, 4K, 8K, 64K, 
128K and 256K), Linux native outperform Xen domain0(this is expected). For 
random write(4K and 8K), both Xen domain0 and Linux native have almost 
identical performance.
Liang

----- Original Message ----- 
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
To: "Liang Yang" <multisyncfe991@xxxxxxxxxxx>; 
<xen-devel@xxxxxxxxxxxxxxxxxxx>
Cc: <ian.pratt@xxxxxxxxxxxx>
Sent: Saturday, October 28, 2006 3:35 PM
Subject: RE: [Xen-devel] Does Xen hypervisor overwrite O_DIRECT setting of 
Linux2.6 kernel?


> Yes, all the testing configurations are the exactly same (both uses
Linux
> kernel 2.6.16.29) and I used Xen 3.0.3. I did several indepedent runs
for
> my testing and the results are consistent, e.g. the performance of
small
> packet
> sequential write under Xen Domain0 outperform Linux native by 10~20%.
>
> Some people told me Xen hypervisor adds addtional layer beyond Linux
I/O
> stack. So O_DIRECT under Xen hypervisor is still valid, but I/O
coalescing
> for small packets are enabled by Xen hypervisor. However, Linux native
> kernel would not do any I/O coalescing when using O_DIRECT.
For IO from dom0 there is no additional layer. There will be no
difference in the coalescing behaviour.

Are you sure you're writing to the same part of the disk in both cases?
You know about disk zoning, right?


Ian

> I hope Xen gurus can give me more explanations about this.
>
> Thanks,
>
> Liang
>
> ----- Original Message -----
> From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
> To: "Liang Yang" <multisyncfe991@xxxxxxxxxxx>;
> <xen-devel@xxxxxxxxxxxxxxxxxxx>
> Cc: <ian.pratt@xxxxxxxxxxxx>
> Sent: Saturday, October 28, 2006 2:34 PM
> Subject: RE: [Xen-devel] Does Xen hypervisor overwrite O_DIRECT
setting of
> Linux2.6 kernel?
>
>
> > The performance data is collected on 8 SAS drives (used as physical
> drives)
> > and IOMeter is used as the benchmark tool. The latest IOMeter
version
> used
> > O_DIRECT. We know, Linux 2.6 kernel starts supporting O_DIRECT which
> makes
> > all I/O requests work around buffer cache. The good thing for
O_DIRECT
> is
> > it
> > reduces the CPU utilization and cache pollution. The bad thing is
> O_DIRECT
> > not only forces all I/O requests become synchronous and no I/O
> coalescing
> > will happen. Thus sequential write of small packets will be impacted
> most.
> > For Xen, however, I believe Xen hypervisor overwrites this O_DIRECT
> setting
> > and maybe it favors better performance over CPU and FSB utilization.
> Thus
> > Xen domain0 can have better write performance than Linux native.
>
> Are you sure you're comparing identical native and dom0 kernel
versions?
> Same drivers and settings?
> Xen does not disable O_DIRECT.
>
> Ian


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