Re: xl create should refuse to share block devices RW between domains

[Jeremy Fitzhardinge <jeremy@goop.org>]

  On 07/27/2010 08:17 AM, Ian Jackson wrote:
> Jeremy Fitzhardinge writes ("xl create should refuse to share block devices RW between domains"):
>>    When creating a domain, "xl create" should fail if a block device is
>> shared RW between domains, like xm create does.
>>
>> I'm not sure how this would be implemented.  Search xenstore for
>> references to the device when setting up a domain?
> Does this safety catch have to be 100% reliable ?  If not then there
> are some useful heuristics: for example, you could see whether the
> device is a devmapper device and if so check its open count.  That
> would catch simultaneously mounting the fs in dom0.

Well, my specific use case is that I have pairs of domain configs, one 
PV, one HVM, referring to the same set of resources.  I want xl create 
to catch when I try and create the PV version of a domain while the HVM 
is still running.

A more comprehensive check would be nice, but just this would be 
useful.  But whatever it does check should be 100% reliable.

> Doesn't the kernel already have some features to try to stop multiple
> conflicting uses of the same block device ?  Perhaps blktap[2] should
> do the same ?

Not in general.  The only interlock the kernel has is that you can't 
change the partition mapping on a device while it is in use.   The 
e2fsprogs do generally try to stop mistakes like fscking a mounted 
filesystem, but it fails to detect if the block device is being used by 
blktap (and that does make quite a bit of a mess, it turns out...).

     J