Re: 4.2.1: Poor write performance for DomU.

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 20/02/13 11:08, 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@crc.id.au> 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? ;)
>
> 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.
>

Sadly that is a hard question to answer, and is driver specific.  I cant
suggest an easy way other than digging into the source.

~Andrew