Re: [Xen-devel] 4.2.1: Poor write performance for DomU.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Feb 20, 2013 at 03:18:46PM +0200, Pasi Kärkkäinen wrote:
> On Wed, Feb 20, 2013 at 10:08:58PM +1100, Steven Haigh wrote:
> > On 20/02/2013 10:06 PM, Andrew Cooper wrote:
> > >On 20/02/13 10:12, Jan Beulich wrote:
> > >>>>>On 20.02.13 at 10:49, Steven Haigh <netwiz@xxxxxxxxx> wrote:
> > >>>My build of Xen 4.2.1 also has all of the recent security advisories
> > >>>patched as well. Although it is interesting to note that downgrading to
> > >>>Xen 4.1.2 made no difference to write speeds.
> > >>Not surprising at all, considering that the hypervisor is only a passive
> > >>library for all PV I/O purposes. You're likely hunting for a kernel side
> > >>regression (and hence the mentioning of the hypervisor version as
> > >>the main factor in the subject is probably misleading).
> > >>
> > >>Jan
> > >
> > >Further to this, do try to verify if your disk driver has changed
> > >recently to use >0 order page allocations for DMA.  If it has, then
> > >speed will be much slower as there will now be the swiotlb cpu-copy
> > >overhead.
> > 
> > Any hints on how to do this? ;)

They are fine. They use the SG DMA API:

konrad@phenom:~/linux/drivers/ata$ grep "dma_map" *
libata-core.c:  n_elem = dma_map_sg(ap->dev, qc->sg, qc->n_elem, qc->dma_dir);

> > 
> > The kernel modules in use for my SATA drives are ahci and sata_mv.
> > There are 6 drives in total on the system.
> > 
> > sda + sdb = RAID1
> > sd[c-f] = RAID6
> > 
> > sda, sdb, sdc and sdd are on the onboard SATA controller (ahci)
> > sde, sdf are on the sata_mv 4x PCIe controller.
> > 
> 
> Can you try using only the disks on the ahci controller?
> 
> sata_mv is known to be buggy and problematic.. 
> I'm not sure if that's the case here, but if you're able to easily 
> try using only ahci, it's worth a shot. 
> 
> -- Pasi
> 
> 
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