Xen users - Mar 2008 - domU dependency-aware xendomains init script?

Is there a smarter xendomains script out there that deals with domU 
dependencies better than the one in the RPMs (CentOS)? 

For example, directory services (eg. LDAP, DNS) are on one domU, and 
other domUs depend on those services to boot correctly.  (Best to solve 
this with redundant services, yes, but let''s say this is a test 
network.)  An even stranger example is a domU whose root boots from an 
NFS, iSCSI or AoE filesystem on another domU.

It would also be nice, for example, to have per-domU settings for 
shutdown (hibernate or init 0?).

This idea is still in a seed stage, so I''m not expressing it very 
clearly.  Any pointers to similar-sounding information, though, are well 
appreciated.  Thanks-

    John


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John Morris wrote:
> Is there a smarter xendomains script out there that deals with domU 
> dependencies better than the one in the RPMs (CentOS)?
> For example, directory services (eg. LDAP, DNS) are on one domU, and 
> other domUs depend on those services to boot correctly.  (Best to 
> solve this with redundant services, yes, but let''s say this is a
test
> network.)  An even stranger example is a domU whose root boots from an 
> NFS, iSCSI or AoE filesystem on another domU.
That''s not a Xen issue: that''s an OS issue. As such, the init
scripts of
the other DomU''s could perform a number of checks to see if the
services
are enabled yet, in theory, but I''d suggest instead a monitoring system
such as Nagios that reports the confused state of the systems and 
reports *that*.
> It would also be nice, for example, to have per-domU settings for 
> shutdown (hibernate or init 0?).
>
> This idea is still in a seed stage, so I''m not expressing it very 
> clearly.  Any pointers to similar-sounding information, though, are 
> well appreciated.  Thanks-
Well, there you''d have to get into the "xendomain" init
script that
RedHat uses. It''s an interesting idea: the information could 
theoretically be contained in a standard Xen init script, and even 
reported to the Xen configurations for migration. But in the short term, 
a sinple text-file with domain names and shutdown states could be 
stuffed in /etc/sysconfig/ or /etx/xen/ for exactly this sort of use.

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Nico,
>> Is there a smarter xendomains script out there that deals with domU 
>> dependencies better than the one in the RPMs (CentOS)?
>> For example, directory services (eg. LDAP, DNS) are on one domU, and 
>> other domUs depend on those services to boot correctly.  (Best to 
>> solve this with redundant services, yes, but let''s say this is
a test
>> network.)  An even stranger example is a domU whose root boots from 
>> an NFS, iSCSI or AoE filesystem on another domU.
> That''s not a Xen issue: that''s an OS issue. As such, the
init scripts
> of the other DomU''s could perform a number of checks to see if the
> services are enabled yet [...]
Yes, you''re right, and I didn''t explain my idea clearly.

When I wrote ''smarter'', I really meant something like the
following:  an
administrator knows that the directory server must be booted first and 
others later.  In a config file somewhere referenced by the xendomains 
init script, probably the same one below that configures per-domU 
settings for shutdown, he inserts the option to, for example, boot the 
dir. server immediately, and then wait 30 seconds to boot the remaining 
servers.  This is a simple example, and there are probably use cases 
where more expression of more complex dependencies would be
desirable.
>> It would also be nice, for example, to have per-domU settings for 
>> shutdown (hibernate or init 0?).
>>
>> This idea is still in a seed stage, so I''m not expressing it
very
>> clearly.  Any pointers to similar-sounding information, though, are 
>> well appreciated.  Thanks-
> Well, there you''d have to get into the "xendomain" init
script that
> RedHat uses. It''s an interesting idea: the information could 
> theoretically be contained in a standard Xen init script, and even 
> reported to the Xen configurations for migration. But in the short 
> term, a sinple text-file with domain names and shutdown states could 
> be stuffed in /etc/sysconfig/ or /etx/xen/ for exactly this sort of use.
So yes, the xendomains init script is where this work would have to be 
done.  The RH xendomains script, and the one shipped with the 
xensource-provided RPMs, are the same IIRC, and both simply fire up all 
configs that exist in /etc/xen/auto (smart enough, at least, to wait a 
few seconds between successive fire-upings).

For now, I am also tempted to stick things into an /etc/sysconfig file 
or perhaps to set variables in the /etc/xen/config scripts.  This is 
just a hack for now, though, and there is probably a smarter way to 
integrate this information into the Xen system somewhere.

    John


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Hi John,
> Yes, you''re right, and I didn''t explain my idea clearly.
>
> When I wrote ''smarter'', I really meant something like the
following:  an
> administrator knows that the directory server must be booted first and
> others later.  In a config file somewhere referenced by the xendomains
> init script, probably the same one below that configures per-domU
> settings for shutdown, he inserts the option to, for example, boot the
> dir. server immediately, and then wait 30 seconds to boot the remaining
> servers.  This is a simple example, and there are probably use cases
> where more expression of more complex dependencies would be desirable.
I rather like your idea, actually.  It seems like something that could 
usefully be done.  A couple of points that may be relevant to some, however:

* This is more complex than probably needed for an initially useful 
proof-of-concept but ideally you''d like a domU to notify dom0 when 
it''s "ready" so that you can start other domains...  Consider
the case where
a server domU usually boots in 30secs but occasionally says
"You''ve mounted
this ext3 filesystem 50 times without fsck, checking all filesystems now, 
please wait three days" ;-)  If you can set up your systems to be more 
deterministic then this is less of an issue!

* For larger, multi-machine Xen deployments you''d need more complexity
(e.g.
manage the dependencies on a cluster manager somewhere) because domUs on one 
machine may depend on those on the other.
> >> It would also be nice, for example, to have per-domU settings for
> >> shutdown (hibernate or init 0?).
> >>
> >> This idea is still in a seed stage, so I''m not expressing
it very
> >> clearly.  Any pointers to similar-sounding information, though,
are
> >> well appreciated.  Thanks-
> >
> > Well, there you''d have to get into the "xendomain"
init script that
> > RedHat uses. It''s an interesting idea: the information could
> > theoretically be contained in a standard Xen init script, and even
> > reported to the Xen configurations for migration. But in the short
> > term, a sinple text-file with domain names and shutdown states could
> > be stuffed in /etc/sysconfig/ or /etx/xen/ for exactly this sort of
use.
>
> So yes, the xendomains init script is where this work would have to be
> done.  The RH xendomains script, and the one shipped with the
> xensource-provided RPMs, are the same IIRC, and both simply fire up all
> configs that exist in /etc/xen/auto (smart enough, at least, to wait a
> few seconds between successive fire-upings).
>
> For now, I am also tempted to stick things into an /etc/sysconfig file
> or perhaps to set variables in the /etc/xen/config scripts.  This is
> just a hack for now, though, and there is probably a smarter way to
> integrate this information into the Xen system somewhere.
In theory, the New Way of handling configurable options like this is to add it 
to the XenAPI and the managed domains facility in Xend.  In practice these 
aren''t used that widely yet and it may be easier for you to hack it up
using
scripting than to delve into Xend''s codebase.

(a comparison point: Managed Domains can be created with an option to make 
Xend auto start them itself, rather than relying on an init script to run 
through /etc/xen/auto run xm create)

Cheers,
Mark

-- 
Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)

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