[Engine-devel] [vdsm] RFC: Writeup on VDSM-libstoragemgmt integration

[Deepak C Shetty]

On 06/25/2012 11:13 PM, Itamar Heim wrote:
> On 06/25/2012 10:14 AM, Deepak C Shetty wrote:
>> On 06/25/2012 07:47 AM, Shu Ming wrote:
>>> On 2012-6-25 10:10, Andrew Cathrow wrote:
>>>>
>>>> ----- Original Message -----
>>>>> From: "Andy Grover" <agrover at redhat.com>
>>>>> To: "Shu Ming" <shuming at linux.vnet.ibm.com>
>>>>> Cc: libstoragemgmt-devel at lists.sourceforge.net,
>>>>> engine-devel at ovirt.org, "VDSM Project Development"
>>>>> <vdsm-devel at lists.fedorahosted.org>
>>>>> Sent: Sunday, June 24, 2012 10:05:45 PM
>>>>> Subject: Re: [vdsm] [Engine-devel] RFC: Writeup on
>>>>> VDSM-libstoragemgmt integration
>>>>>
>>>>> On 06/24/2012 07:28 AM, Shu Ming wrote:
>>>>>> On 2012-6-23 20:40, Itamar Heim wrote:
>>>>>>> On 06/23/2012 03:09 AM, Andy Grover wrote:
>>>>>>>> On 06/22/2012 04:46 PM, Itamar Heim wrote:
>>>>>>>>> On 06/23/2012 02:31 AM, Andy Grover wrote:
>>>>>>>>>> On 06/18/2012 01:15 PM, Saggi Mizrahi wrote:
>>>>>>>>>>> Also, there is no mention on credentials in any part of the
>>>>>>>>>>> process.
>>>>>>>>>>> How does VDSM or the host get access to actually modify the
>>>>>>>>>>> storage
>>>>>>>>>>> array? Who holds the creds for that and how? How does the user
>>>>>>>>>>> set
>>>>>>>>>>> this up?
>>>>>>>>>> It seems to me more natural to have the oVirt-engine use
>>>>>>>>>> libstoragemgmt
>>>>>>>>>> directly to allocate and export a volume on the storage array,
>>>>>>>>>> and
>>>>>>>>>> then
>>>>>>>>>> pass this info to the vdsm on the node creating the vm. This
>>>>>>>>>> answers
>>>>>>>>>> Saggi's question about creds -- vdsm never needs array
>>>>>>>>>> modification
>>>>>>>>>> creds, it only gets handed the params needed to connect and use
>>>>>>>>>> the
>>>>>>>>>> new
>>>>>>>>>> block device (ip, iqn, chap, lun).
>>>>>>>>>>
>>>>>>>>>> Is this usage model made difficult or impossible by the current
>>>>>>>>>> software
>>>>>>>>>> architecture?
>>>>>>>>> what about live snapshots?
>>>>>>>> I'm not a virt guy, so extreme handwaving:
>>>>>>>>
>>>>>>>> vm X uses luns 1& 2
>>>>>>>>
>>>>>>>> engine -> vdsm "pause vm X"
>>>>>>> that's pausing the VM. live snapshot isn't supposed to do so.
>>>>>> Tough we don't expect to do a pausing operation to the VM when live
>>>>>> snaphot is undergoing, the VM should be blocked on the access to
>>>>>> specific luns for a while. The blocking time should be very short
>>>>>> to
>>>>>> avoid the storage IO time out in the VM.
>>>>> OK my mistake, we don't pause the VM during live snapshot, we block
>>>>> on
>>>>> access to the luns while snapshotting. Does this keep live snapshots
>>>>> working and mean ovirt-engine can use libsm to config the storage
>>>>> array
>>>>> instead of vdsm?
>>>>>
>>>>> Because that was really my main question, should we be talking about
>>>>> engine-libstoragemgmt integration rather than vdsm-libstoragemgmt
>>>>> integration.
>>>> for snapshotting wouldn't we want VDSM to handle the coordination of
>>>> the various atomic functions?
>>>
>>> I think VDSM-libstoragemgmt will let the storage array itself to make
>>> the snapshot and handle the coordination of the various atomic
>>> functions. VDSM should be blocked on the following access to the
>>> specific luns which are under snapshotting.
>>
>> I kind of agree. If snapshot is being done at the array level, then the
>> array takes care of quiesing the I/O, taking the snapshot and allowing
>> the I/O, why does VDSM have to worry about anything here, it should all
>> happen transparently for VDSM, isnt it ?
>
> I may be misssing something, but afaiu you need to ask the guest to 
> perform the quiesce, and i'm sure the storage array can't do that.

No, you are not, I missed it. After Tony & Shu Ming's reply, I realised 
that the guest has to quiese the I/O before VDSM can ask storage array 
to take the snapshot.
>
>