Re: [Xen-users] File-based domU - Slow storage write since xen 4.8

[Benoit Depail <benoit.depail@nbs-system.com>]

On 08/01/2017 11:48 AM, Roger Pau Monné wrote:
> On Fri, Jul 28, 2017 at 04:50:27PM +0200, Benoit Depail wrote:
>> On 07/26/17 00:25, Keith Busch wrote:
>>> On Fri, Jul 21, 2017 at 07:07:06PM +0200, Benoit Depail wrote:
>>>> On 07/21/17 18:07, Roger Pau Monné wrote:
>>>>>
>>>>> Hm, I'm not sure I follow either. AFAIK this problem came from
>>>>> changing the Linux version in the Dom0 (where the backend, blkback is
>>>>> running), rather than in the DomU right?
>>>>>
>>>>> Regarding the queue/sectors stuff, blkfront uses several blk_queue
>>>>> functions to set those parameters, maybe there's something wrong
>>>>> there, but I cannot really spot what it is:
>>>>>
>>>>> http://elixir.free-electrons.com/linux/latest/source/drivers/block/xen-blkfront.c#L929
>>>>>
>>>>> In the past the number of pages that could fit in a single ring
>>>>> request was limited to 11, but some time ago indirect descriptors
>>>>> where introduced in order to lift this limit, and now requests can
>>>>> have a much bigger number of pages.
>>>>>
>>>>> Could you check the max_sectors_kb of the underlying storage you are
>>>>> using in Dom0?
>>>>>
>>>>> Roger.
>>>>>
>>>> I checked the value for the loop device as well
>>>>
>>>> With 4.4.77 (bad write performance)
>>>> $ cat /sys/block/sda/queue/max_sectors_kb
>>>> 1280
>>>> $ cat /sys/block/loop1/queue/max_sectors_kb
>>>> 127
>>>>
>>>>
>>>> With 4.1.42 (normal write performance)
>>>> $ cat /sys/block/sda/queue/max_sectors_kb
>>>> 4096
>>>> $ cat /sys/block/loop1/queue/max_sectors_kb
>>>> 127
>>>
>>> Thank you for the confirmations so far. Could you confirm performance dom0
>>> running 4.4.77 with domU running 4.1.42, and the other way around? Would
>>> like to verify if this is just isolated to blkfront.
>>>
>> Hi,
>>
>> I've ran the tests, and I can tell that the domU kernel version have no
>> influence on the performance.
>>
>> Dom0 with 4.4.77 always shows bad performance, wether the domU runs
>> 4.1.42 or 4.4.77.
>>
>> Dom0 with 4.1.42 always shows good performance, wether the domU runs
>> 4.1.42 or 4.4.77.
> 
> Hello,
> 
> I haven't yet got time to look into this sadly. Can you please try to
> use fio [0] in order to run the tests against the loop device in Dom0?
> 
> If possible, could you test several combinations of block sizes, I/O
> sizes and I/O depths?
> 
> Thanks, Roger.
> 
> [0] http://git.kernel.dk/?p=fio.git;a=summary
> 

Ok I'll give a try later when I have more time. Probably next week.

Thanks,

-- 
Benoit Depail
Senior Infrastructures Architect
NBS System