Xen devel - Aug 2012 - Bug#666135: XenStore tdb vs. reboot

Hello,

I noticed a strace delay when running "virsh list" on my Xen-4.1.3
test
server. On further investigation I noticed that in XenStore there were 
multiple /vm/$UUID entries suffixed by -$VERSION, especially I hat 16 entries 
for my dom0. For each reboot of the host I get a new entry.

After disabling Xend I get the following output after a
reboot:
> root at xen5:~# xenstore-ls /local/domain/0
> name = "Domain-0"
If I then start Xend, I get more data added:
> root at xen5:~# /etc/init.d/xend start
> Starting xen management daemon: xend.
> root at xen5:~# xenstore-ls /local/domain/0
> vm = "/vm/00000000-0000-0000-0000-000000000000-2"
> device = ""
> control = ""
>  platform-feature-multiprocessor-suspend = "1"
> error = ""
> memory = ""
>  target = "3400192"
> guest = ""
> hvmpv = ""
> data = ""
> cpu = ""
>  1 = ""
>   availability = "online"
>  0 = ""
>   availability = "online"
> description = ""
> console = ""
>  limit = "1048576"
>  type = "xenconsoled"
> domid = "0"
> name = "Domain-0"
Notice that I now have vm=$UUID-2.

If you just restart Xend, the problem does not show; you need to actually 
reboot to host, so that /local/domain/0/vm does not exist.

I found commit
<http://xenbits.xen.org/hg/xen-unstable.hg/rev/265950e3df69>
which added the suffix handling, which I never noticed before.

I found the same issue reported at 
<http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=666135>.

For comparison I just installed a fresh Debian Wheezy rc1 and noticed that the 
XenStore tdb there is locates in /var/_run_/xenstore/tdb instead 
of /var/_lib_/xenstore/tdb. Since /var/run/ lives on a tempfs, it's nuked on
every reboot and thus the problem doesn't exist there.

Is the XenStore tdb supposed to survive a reboot or must it be cleared between 
each reboot?

Sincerely
Philipp
-- 
Philipp Hahn           Open Source Software Engineer      hahn at univention.de
Univention GmbH        be open.                       fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
                                                   http://www.univention.de/
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On Fri, 2012-08-24 at 16:00 +0100, Philipp Hahn wrote:
> Is the XenStore tdb supposed to survive a reboot or must it be cleared
> between each reboot? 
Stuff in xenstore is not required to survive a reboot and clearing it on
boot is a good best practice, since it avoids bugs in the C
xenstored ;-)

The ocaml xenstore doesn't even have an on disk DB by default.

Ian.

On Fri, Aug 24, 2012 at 04:05:56PM +0100, Ian Campbell
wrote:
> On Fri, 2012-08-24 at 16:00 +0100, Philipp Hahn wrote:
> > Is the XenStore tdb supposed to survive a reboot or must it be cleared
> > between each reboot? 
> 
> Stuff in xenstore is not required to survive a reboot and clearing it on
> boot is a good best practice, since it avoids bugs in the C
> xenstored ;-)
Many installations that are using C xenstored mount /var/lib/xenstored
as a tmpfs. It seems that a proposed patch [1] to the Linux init
scripts (pre-xencommons) wasn''t applied.
> The ocaml xenstore doesn''t even have an on disk DB by default.
> 
> Ian.
Matt

[1] http://lists.xen.org/archives/html/xen-devel/2010-01/msg00667.html