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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Jul 21, 2017 at 11:53:33AM -0400, Keith Busch wrote:
> On Fri, Jul 21, 2017 at 12:19:39PM +0200, Benoit Depail wrote:
> > On 07/20/17 19:36, Keith Busch wrote:
> > > 
> > > As a test, could you throttle the xvdb queue's max_sectors_kb? If I
> > > followed xen-blkfront correctly, the default should have it set to 44.
> > > Try setting it to 40.
> > > 
> > >   echo 40 > /sys/block/xvdb/queue/max_sectors_kb
> > > 
> > 
> > The default value on my domU is 128.
> > 
> > I ran a couple of tests with different values, starting from 40 and up
> > to 128, clearing the cache between each tests.
> > 
> > The only value that showed the issue is 128. Even setting max_sectors_kb
> > to 127 is enough to get normal behaviour.
> 
> Ok, I don't quite follow how it's initialized to 128k, but your
> results appear to confirm the default settings are not optimal for the
> interface. The patch you identified just submits commands to the max
> size the driver asked to use. If the largest size tanks performance,
> I think xen-blkfront should register a smaller transfer size, or maybe
> some other constraint needs to be refined.
> 
> Roger,
> 
> I'm a bit out of my depth here in the xen code. Is this something you
> may be able to help clarify?

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.

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