[openstack-dev] [nova][libvirt] - 'nova reboot' causes console-log truncated
suro.patz at gmail.com
Thu Nov 13 21:55:06 UTC 2014
If we issue 'nova reboot <server>', we get to have the console output of
the latest bootup of the server only. The console output of the previous
boot for the same server vanishes due to truncation. If we do reboot
from within the VM instance [ #sudo reboot ], or reboot the instance
with 'virsh reboot <instance>' the behavior is not the same, where the
console.log keeps increasing, with the new output being appended.
This loss of history makes some debugging scenario difficult due to lack
of information being available.
Please point me to any solution/blueprint for this issue, if already
planned. Otherwise, please comment on my analysis and proposals as
solution, below -
Nova's libvirt driver on compute node tries to do a graceful restart of
the server instance, by attempting a soft_reboot first. If soft_reboot
fails, it attempts a hard_reboot. As part of soft_reboot, it brings down
the instance by calling shutdown(), and then calls createWithFlags() to
bring this up. Because of this, qemu-kvm process for the instance gets
terminated and new process is launched. In QEMU, the chardev file is
opened with O_TRUNC, and thus we lose the previous content of the
On the other-hand, during 'virsh reboot <instance>', the same qemu-kvm
process continues, and libvirt actually does a
qemuDomainSetFakeReboot(). Thus the same file continues capturing the
new console output as a continuation into the same file.
[Proposals for solution]
1. NOVA, driven by certain configuration, will backup the console file,
before creating the domain, during reboot scenario. viz. doing a backup
of console.log as console.log.0. How many such backups of log-file to
keep, what can be the maximum size of the file, 'logrotate` to be used
or not - all these can come to NOVA as configuration parameter.
Pros - As NOVA libvirt driver is not using libvirt's reboot()
functionality knowingly, this problem can be better addressed from the
Cons - NOVA's libvirt layer building awareness of the console files
is not clean from modularity.
2. virDomainCreateWithFlags() will have a new flag value to indicate
logs to be appended instead of truncated, if FILE option is used. This
config will be passed to QEMU, while spawning the process.
- Changes will be not in OpenStack Code, but in libvirt and QEMU.
Cons - We may have to do the similar implementation for all the
drivers of libvirt.
Pros - This feature's use-case is there in case of 'virsh shutdown
<instance>', followed by a 'virsh start <instance>' too.
# tail -f
[ OK ] Started udev Coldplug all Devices.
[ OK ] Started Create static device nodes in /dev.
Starting udev Kernel Device Manager...
[ 36.938075] EXT4-fs (vda1): re-mounted. Opts: (null)
[ OK ] Started Remount Root and Kernel File Systems.
Starting Load/Save Random Seed...
[ OK ] Reached target Local File Systems (Pre).
Starting Configure read-only root support...
[ OK ] Started Load/Save Random Seed.
[ 0.000000] Initializing cgroup subsys cpuset
[ 0.000000] Initializing cgroup subsys cpu
[ 0.000000] Initializing cgroup subsys cpuacct
[ 0.000000] Linux version 3.11.10-301.fc20.x86_64
(mockbuild at bkernel01.phx2.fedoraproject.org) (gcc version 4.8.2 20131017
(Red Hat 4.8.2-1) (GCC) ) #1 SMP Thu Dec 5 14:01:17 UTC 2013
[ 0.000000] Command line: ro
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