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Re: [Xen-devel] xl: multiple domain with the same name allowed?

To: Andre Przywara <andre.przywara@xxxxxxx>
Subject: Re: [Xen-devel] xl: multiple domain with the same name allowed?
From: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
Date: Fri, 10 Sep 2010 09:45:45 +0100
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>
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On Fri, 2010-09-10 at 09:03 +0100, Andre Przywara wrote:
> Hi,
> I realized that the xl tool allows to create multiple domains with the 
> same name:
> # xl create ttylinux.xl
> # xl create ttylinux.xl
> # xl list
> Name                        ID   Mem VCPUs      State   Time(s)
> Domain-0                     0  5498     4     r-----    1647.8
> TTYLinux-NUMA               22  2043     4     -b----      29.9
> TTYLinux-NUMA               23  2043     4     r-----      21.3
> xm only shows one domain, it also refuses to start another instance (in 
> opposite to xl)
> # xm list
> Name                        ID   Mem VCPUs      State   Time(s)
> Domain-0                     0  5498     4     r-----   1665.0
> TTYLinux-NUMA               22  2043     4     -b----    133.1
> # xm create ttylinux.xl
> Using config file "./ttylinux.xm".
> Error: Domain 'TTYLinux-NUMA' already exists with ID '22'
> Is the xl behavior intended or just a bug?

It's a bug (or at best a missing feature).

While creating multiple domains with the same name is only confusing to
the user (and therefore it would be better, I think, for xl to enforce
uniqueness by default if possible) a more serious issue is allowing
multiple domains to be started which refer to the same storage since
this can lead to data corruption. (the obvious way to do this
accidentally is starting same domain twice, or via a typo in your
configuration file)

There was some discussion of this on xen-devel several weeks back but I
don't think anyone quite got to the bottom of why the locking in the
block backend hotplug scripts wasn't preventing the second and
subsequent domains using a given storage backend from connecting to
their devices, which would prevent damage from occurring. Really xl
ought to be capable of detecting this situation before even starting a
domain, which is what I think xend does. (perhaps this is harder with xl
due to the lack of an overarching daemon for coordination).


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