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Re: [Xen-devel] Degregated I/O Performance since 3.4 - Regression in 3.4?

On Mon, 23 Apr 2012, Tobias Geiger wrote:
> Hello!
> i noticed a considerable drop in I/O Performance when using 3.4 (rc3 and rc4 
> tested) as Dom0 Kernel;
> With 3.3 i get over 100mb/s in a HVM DomU (win64) with PV Drivers 
> (gplpv_Vista2008x64_0.11.0.357.msi); 
> With 3.4 it drops to about a third of that.
> Xen Version is xen-unstable: 
> xen_changeset          : Tue Apr 17 19:13:52 2012 +0100 25209:e6b20ec1824c
> Disk config line is:
> disk = [ '/dev/vg_ssd/win7system,,hda' ] 
> - it uses blkback.

I fail to see what could be the cause of the issue: nothing on the
blkback side should affect performances significantly.
You could try reverting the four patches to blkback that were applied
between 3.3 and 3.4-rc3 just to make sure it is not a blkback

$ git shortlog v3.3..v3.4-rc3 drivers/block/xen-blkback
Daniel De Graaf (2):
      xen/blkback: use grant-table.c hypercall wrappers
      xen/blkback: Enable blkback on HVM guests

Konrad Rzeszutek Wilk (2):
      xen/blkback: Squash the discard support for 'file' and 'phy' type.
      xen/blkback: Make optional features be really optional.

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