[et-mgmt-tools] Virtinst and blktap
Cole Robinson
crobinso at redhat.com
Thu Jun 4 20:58:51 UTC 2009
Cole Robinson wrote:
> Michael DeHaan wrote:
>> Cole Robinson wrote:
>>> 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
>>>
>> Yeah, I can't keep up very well when stuff doesn't happen there :)
>>
>> Should I make the fix in koan also or do you want to move it?
>>
>> --Michael
>>
>
> I'll take a look at moving it into the library, but for back compat it
> wouldn't hurt to set the default in koan. The virt-install code is:
>
> http://hg.et.redhat.com/cgi-bin/hg-virt.cgi/applications/virtinst--devel/file/5d6dc8af58b5/virt-install#l209
>
This is now upstream: we will set the 'tap' default if installing a PV
guest:
http://hg.et.redhat.com/cgi-bin/hg-virt.cgi/applications/virtinst--devel/rev/ab66c26436cf
If creating a standalone VirtualDisk instance though, the default won't
be set, since we don't know what type of guest it will be attached to.
- Cole
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