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

[Keith Busch <keith.busch@intel.com>]

On Fri, Jul 21, 2017 at 07:07:06PM +0200, Benoit Depail wrote:
> On 07/21/17 18:07, Roger Pau Monn� 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.