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Re: [et-mgmt-tools] Virtinst and blktap



Michael DeHaan wrote:
> Chris Lalancette wrote:
>> Michael DeHaan wrote:
>>   
>>> I had a user question about how they can specify blktap for Xen in 
>>> Cobbler instead of the file driver (for performance reasons) ... well, 
>>> it turns out they can't (yet).
>>>
>>> Under what conditions can we use blktap, and is that advisable?   (I had 
>>> thought this might work like virtio, in which case, passing in the os 
>>> version uses a table, but it seems this may only actually depend on the 
>>> host and has no guest limitations?)
>>>     
>> That's exactly right.  It's a property of how the host OS accesses the disk on
>> behalf of the domain, not of how the disk is presented to the domain.  In fact,
>> for Xen PV domains, you should *only* ever use blktap (i.e. tap:aio:); direct
>> file: access can corrupt your guest domains on host crash.  The situation with
>> FV domains is more complicated, but there, you can use file:, since it properly
>> does flushing.
>>
>>   
> 
> 
>>> It looks like from the virtinst code I could just call 
>>> utils.is_blktap_capable() from my libvirt-using application and if so, 
>>> then pass the driver=DRIVER_TAP options to virstinst.
>>>
>>> Good idea?  Bad idea?
>>>     
>> The only reasonable default for Xen PV domains, in my opinion.
>>
>>   
> 
> Thanks.
> 
> FWIW the default behavior of python-virtinst seems to be to /not/ use 
> blktap, and should probably be fixed then if there's a potential 
> corruption problem.
> 
> I haven't tested command line virt-install to see what it does.
> 

That default is actually set in virt-install and virt-manager, but
probably should have been moved into the virtinst library a long time ago.

- COle


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