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

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.


I'm a bit out of my depth here in the xen code. Is this something you
may be able to help clarify?


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