[CentOS-virt] (long) kvm and virt-manager not ready for daily usage

Farkas Levente lfarkas at bppiac.hu
Sat Sep 8 11:15:04 UTC 2007

in the last 2 weeks we play a lot with our new server which we but to a
our virtual server for the development and collect some very subjective
experience. we use kvm and virt-manager, but sometimes i'm not really
sure about the whether it's kvm or virt-manager problem so i collect
them together. our system:
MB: Intel S3000AHV
CPU: Intel Core 2 Quad CPU Q6600 @ 2.40GHz
CentOS 5 x86-64 host system with:

kvm is not ready for production use for many reason:
it can't reboot which imho a very basic feature, what's more can't even
shutdown/poweroff. the centos i386 guest are not able to shutdown on the
x86_64 host (strange the x86_64 centos and the i586 mandrake-9 are able
to shutdown) or what's seems to be more likely it's a random think.

can't give 2GB ram to any guests not even 2, but 1.9999 is ok. what's
more i've to discover this in the hard way virt-manager gives some
strange error message (anyway none of the virt-manager's error message
are useful, just strange stack trace without any kind of useful info).

the host see as i've 4 cpu. i've got a change to gives more cpu to the
guest, what's more they starts, but after a few minutes running the
system crash. not just the guest os but the host os crash without any
kind of info, log or any useful info what was the cause of it, but hard
reset helps:-((( first of all it's serious problem, even if it's a known
bug and documented somewhere (but there is not any kind of docs about
neither kvm nor virt-manager/libvirt and what i can find that very
limited), since all of the new hvm cpu (which is required for kvm) has
more core, so the guest can't use the real power of the cpu this means
i've to put a lots of guests with one virtual cpu to the host or
currently can't exploit these cpus.

libvirt not start the guest when the host started, not saved the state
of the guests when stopped, when restarted all guest get into shutdown
state. may be it's just libvirt but, but if kvm not able to save it's
guest's state that it's a real pain. this behavior not acceptable in a
real environment.

in virt-manager can't modify virtual cpu number of the guests
(virt-manager crash with another unusable stacks trace). it i modify the
xml config file by hand i've to restart libvirt in order to read it.
even if i just modify one guest's config, but as if i restart libvirtd
the all other guest will be killed it's another nice feature.

from vit-manager i can't assign swap partitions (and any other device
to the guests) during guest creation. i precreate lvm partition on the
host for root and swap to all of our guests (about 10) but it's a real
pain to reload libvirtd (which kills all guests) to add a new disk to
the guest which can be used as a swap or other partition. what's more
it'd be useful to add swap partition during install. the same thing
apply to cdrom too.

if i made a mistake in the libvirtd xml config file, then i don't get
any kind of error message just libvirt don't start the given guest. no
error message service seems to start without error and don't know what
was the reason that the guest not start (sometimes because syntactic
error in the config file, sometimes kvm/qemu not support it eg more then
2gb mem). there are unwritten unix/linux tradition eg if there are error
during a service startup report it some log file. if a service can't
start without error then it not start at all eg. if i've apache with 5
virtual host but one of it's config has error then the whole httpd not
started. it'd have to be the same with libvirt too.

libvirt config file has all the features as qemu command line (if not it
can't be used by advanced users)? but i don't know since there is not a
reference docs for the config file.

there is not a detailed reference docs about libvirtd config (and at the
same time the gui not contains all the settings). for the the xml format
is a bit redundant eg. "source dev", "target dev" why not just source
and target, inside interface type='bridge' 'source bridge' why not just
source (i've to modify two place when i'd like to switch from network to
bride. ok these are just small cosmetic thing just mention.

when create a new guest in virt-manager and reach the last page and
press finish then most of the time (2 times from 3) i've got an error
something like "can't connect to qemu" after i press ok and press again
finish then it's able to create the new host (so always have to retry it).

in virt-manager when a guest is shutdown and double-click on the guest
and try to start it it's not possible. you've to select the guest in the
main windows, right-click and start it.

the new vnc based console is very unstable or just virt-manager not able
to connect it. many times i double click on a guest in virt-manager i
got the window can't connect to console while the guest are running.
that's why i like to see again the serial console.

we've got other virtual servers with xen and openvz, with has other
problems but works. my conclusion is that currently kvm, libvirtd,
virt-manager (et-mgmt-tools) seems very promising. but currently just in
a very alpha stage using even in non mission critical but production
environment still a very 'brave' decision. xen has a few years vantage,
but it's at least usable in real environment.
i'm on the these list and see what kind of problem you're working which
is great but at the same it'd be useful to fix the most basic problems
with these tools.

just my 2c to these community.

  Levente                               "Si vis pacem para bellum!"

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