[et-mgmt-tools] Virtinst and blktap

[Michael DeHaan]

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.