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

[WebDawg <webdawg@xxxxxxxxx>]

On Thu, Jul 13, 2017 at 12:42 PM, Benoit Depail <benoit.depail@xxxxxxxxxxxxxx> wrote:

Hi,

At my company, we are using file-based domU for our hosting infrastructures.

We have no issues on Xen 4.4 (Debian Jessie). However we are concerned
by this aging platform and are testing Xen 4.8 (Debian Stretch).

The problem is that our storage writing speed are about the third of
what they used to be.

On xen 4.4, the domU and the dom0 can easily reach a steady 100MB/s when
writing to the image file.

On xen 4.8, the domU is stuck at about 30MB/s whereas the dom0 can still
reach about 100MB/s.

The domU is the same in both case, I just switch the dom0 OS.

I doubt that the cache have anything to do with it since the image file
is hosted on a NFS share, and I can see the network traffic reach
800Mb/s when everything is fine.

I could also reproduce this behaviour on a stock alpine linux based
dom0, running xen 4.8. So it seems our debian installation is not in cause.

There are no changes in reading speed.


What could explain this behaviour ?

--
Benoit Depail
Senior Infrastructures Architect
NBS System

Now this is very interesting.   Is it the same hardware, switches, nics and all?

I might start with looking at NFS.  The new kernel might require some different settings to match what the old did automatically.  I do not know what has changed in NFS.

You are saying you tested NFS speed on a dom0?  I know dom0 is more like a domU then most think but what tests do you get out of that?

I would start with direct NFS speeds and move forward through the storage stack.  I would do an iperf test too.

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