[Pkg-xen-devel] Bug#452721: #452721 moreinfo?

[Elliott Mitchell]

I'm surprised #452721 is tagged moreinfo since it seems simple, but that
may depend on installation capability.

Note, I am not the original reporter, so I might actually be observing
something distinct.  I doubt this, but I cannot be certain.


Issue is this, a hypervisor machine could have tens or even hundreds of
VMs.  There could be ordering dependencies during startup and shutdown.

Notably there are core services, such as LDAP, DHCP, fileserver and DNS.
Often these need to be up before anything else and they may need to come
up in a particular order.  Most often the LDAP server (which can be a
distinct VM) needs to be up first.  Meanwhile for downtimes, a fileserver
(which can also be a VM) needs to go down last.

During a full downtime when all VMs were fully shut down, this effect
can be achieved by including numbers in the filename.  Say
/etc/xen/auto/0_ldap.cfg, /etc/xen/auto/1_fileserver.cfg,
/etc/xen/auto/9_everything_else.cfg.

If the hypervisor is rebooted and VMs are saved to /var/lib/xen/save;
they will be paused in identifier order, but saved by domain name.  When
scanning /var/lib/xen/save, `xendomains` goes by filename which means VMs
are restored in a distinct (and often problematic) order.


A minimal solution would be for `xendomains` to save VMs in
/var/lib/xen/save <domId>-<name> and then use `sort -n` during restore.

A better approach would be to have a LSB style header specifying
dependencies to flag VMs which should be saved or shutdown late, and VMs
which should be saved or shutdown early.

A ridiculous overkill solution might be to turn the /etc/xen/*.cfg files
into full init scripts.  This could be done by having a script which
understood domain configuration files well enough to identify the
name/UUID and then start/stop the domain as specified by $1.  Use that
script as the interpreter (#! line), then it could find the configuration
via $0.  Then normal init script handling tools could take care of
ordering.

(geeze, that really does actually seem kind of like a semi-workable
solution despite seeming rather crazy at first)


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