[et-mgmt-tools] Virtinst and blktap
Cole Robinson
crobinso at redhat.com
Thu May 28 13:06:13 UTC 2009
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
- Cole
More information about the et-mgmt-tools
mailing list