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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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