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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2010-09-10 at 11:34 +0100, Marc - A. Dahlhaus wrote:
> Am Freitag, den 10.09.2010, 09:45 +0100 schrieb Ian Campbell:
> > 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)
> 
> Wouldn't this prevent the "shared block device with a cluster aware
> filesystem on it" use-case?

Yes, it would.

I think it would be better to try and prevent disaster in the common
case (which is not shared writeable block devices) and allow a
configuration override for users who have uncommon needs like this.

[...]
> If he fails, he will learn why and will not do the same mistake again.

That's cold comfort when you've just trashed a filesystem because of a
typo in a configuration file.

Ian.



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