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Dear Fabrice, please try to avoid personal feelings statements about issues and describe the steps you did to reproduce this print. 
Vdsm reports DEBUG level logs by default. you can modify it in /etc/vdsm/logger.conf file. second, I think it relates to the same issue specified in Bug 1324731, but I'll check it more deeply, same for the unicode odd characters that you see. steps for reproduction will help. did you upgrade vdsm from certain version? can you share vdsm.log as well?

Edy, please check. upgrade.log relates to network flows, can you investigate it?

I have just reformatted and reinstalled from scratch an host after removed it from ovirt, and got the exact same message:

MainThread::DEBUG::2016-04-25 16:31:20,637::utils::689::root::(execCmd) SUCCESS: <err> = 'cp: cannot stat \xe2\x80\x98/var/run/vdsm/netconf\xe2\x80\x99: No such file or directory\n'; <rc> = 0

So upgrade.log is generated even before starting vdsm, as the first line of vdsm.log is :
MainThread::INFO::2016-04-25 16:31:22,204::vdsm::162::vds::(run) (PID: 6628) I am the actual vdsm 4.17.26-0.el7.centos XXXX (3.10.0-327.13.1.el7.x86_64)

The full upgrade.log is:
MainThread::DEBUG::2016-04-25 16:31:20,394::upgrade::90::upgrade::(apply_upgrade) Running upgrade upgrade-unified-persistence
MainThread::DEBUG::2016-04-25 16:31:20,414::libvirtconnection::161::root::(get) trying to connect libvirt
MainThread::DEBUG::2016-04-25 16:31:20,624::utils::671::root::(execCmd) /usr/bin/taskset --cpu-list 0-31 /sbin/ip route show to 0.0.0.0/0 table main (cwd None)
MainThread::DEBUG::2016-04-25 16:31:20,626::utils::689::root::(execCmd) SUCCESS: <err> = ''; <rc> = 0
MainThread::DEBUG::2016-04-25 16:31:20,626::unified_persistence::46::root::(run) upgrade-unified-persistence upgrade persisting networks {} and bondings {}
MainThread::INFO::2016-04-25 16:31:20,626::netconfpersistence::187::root::(_clearDisk) Clearing /var/run/vdsm/netconf/nets/ and /var/run/vdsm/netconf/bonds/
MainThread::DEBUG::2016-04-25 16:31:20,627::netconfpersistence::195::root::(_clearDisk) No existent config to clear.
MainThread::INFO::2016-04-25 16:31:20,627::netconfpersistence::187::root::(_clearDisk) Clearing /var/run/vdsm/netconf/nets/ and /var/run/vdsm/netconf/bonds/
MainThread::DEBUG::2016-04-25 16:31:20,627::netconfpersistence::195::root::(_clearDisk) No existent config to clear.
MainThread::INFO::2016-04-25 16:31:20,627::netconfpersistence::131::root::(save) Saved new config RunningConfig({}, {}) to /var/run/vdsm/netconf/nets/ and /var/run/vdsm/netconf/bonds/
MainThread::DEBUG::2016-04-25 16:31:20,627::utils::671::root::(execCmd) /usr/bin/taskset --cpu-list 0-31 /usr/share/vdsm/vdsm-store-net-config unified (cwd None)
MainThread::DEBUG::2016-04-25 16:31:20,637::utils::689::root::(execCmd) SUCCESS: <err> = 'cp: cannot stat \xe2\x80\x98/var/run/vdsm/netconf\xe2\x80\x99: No such file or directory\n'; <rc> = 0
MainThread::DEBUG::2016-04-25 16:31:20,637::upgrade::51::upgrade::(_upgrade_seal) Upgrade upgrade-unified-persistence successfully performed

I don't know if it's linked, but locale for this server is C:
$ locale
LANG=C
LC_CTYPE="C"
LC_NUMERIC="C"
LC_TIME="C"
LC_COLLATE="C"
LC_MONETARY="C"
LC_MESSAGES="C"
LC_PAPER="C"
LC_NAME="C"
LC_ADDRESS="C"
LC_TELEPHONE="C"
LC_MEASUREMENT="C"
LC_IDENTIFICATION="C"
LC_ALL=

Fabrice,

Let me translate these logs for you:

MainThread::DEBUG::2016-04-25 16:31:20,627::utils::671::root::(execCmd) /usr/bin/taskset --cpu-list 0-31 /usr/share/vdsm/vdsm-store-net-config unified (cwd None)

Here we run the command:

    /usr/bin/taskset --cpu-list 0-31 /usr/share/vdsm/vdsm-store-net-config unified

MainThread::DEBUG::2016-04-25 16:31:20,637::utils::689::root::(execCmd) SUCCESS: <err> = 'cp: cannot stat \xe2\x80\x98/var/run/vdsm/netconf\xe2\x80\x99: No such file or directory\n'; <rc> = 0

Here the command terminated with exit code 0 - so this is a SUCCESS.

The command wrote some data to stderr:

    'cp: cannot stat \xe2\x80\x98/var/run/vdsm/netconf\xe2\x80\x99: No such file or directory\n'

We log stderr of all commands - why we do this is good question. Maybe the
the contents of stderr was useful for debugging in the past.

What are the xe2\x80\x98 - good question - our networking team responsible for
this command (/usr/share/vdsm/vdsm-store-net-config) should answer this.

Why we log DEBUG message? it help us to support the system. We plan to move to 
INFO level by default in future version.

Edi, can you take a look at the error message from vdsm-store-net-config?

Nir

In vdsm-store-net-config, running-config is persisted by copying it to a dedicated location.
If for some edge case scenario, the running-config is absent an error message appeared in the log but without failing the script run.

Q1: Do we still need the upgrade step in 4.x? (it fails in that scenario)
Q2: Are we ok with the missing running config? If we are, the proposed patch handles it.

The patch that avoids the mentioned error message has been merged long ago, closing this bug.