[ovirt-devel] vdsm disabling logical volumes

Wed May 7 07:21:28 UTC 2014

On 05/05/2014 03:19 PM, Nir Soffer wrote:
> ----- Original Message -----
>> From: "Jiri Moskovcak" <jmoskovc at redhat.com>
>> To: "Nir Soffer" <nsoffer at redhat.com>
>> Cc: devel at ovirt.org, "Federico Simoncelli" <fsimonce at redhat.com>, "Allon Mureinik" <amureini at redhat.com>, "Greg
>> Padgett" <gpadgett at redhat.com>
>> Sent: Monday, May 5, 2014 3:44:21 PM
>> Subject: Re: [ovirt-devel] vdsm disabling logical volumes
>>
>> On 05/05/2014 02:37 PM, Nir Soffer wrote:
>>> ----- Original Message -----
>>>> From: "Jiri Moskovcak" <jmoskovc at redhat.com>
>>>> To: "Nir Soffer" <nsoffer at redhat.com>
>>>> Cc: devel at ovirt.org, "Federico Simoncelli" <fsimonce at redhat.com>, "Allon
>>>> Mureinik" <amureini at redhat.com>, "Greg
>>>> Padgett" <gpadgett at redhat.com>
>>>> Sent: Monday, May 5, 2014 3:16:37 PM
>>>> Subject: Re: [ovirt-devel] vdsm disabling logical volumes
>>>>
>>>> On 05/05/2014 12:01 AM, Nir Soffer wrote:
>>>>> ----- Original Message -----
>>>>>> From: "Jiri Moskovcak" <jmoskovc at redhat.com>
>>>>>> To: "Nir Soffer" <nsoffer at redhat.com>
>>>>>> Cc: devel at ovirt.org
>>>>>> Sent: Sunday, May 4, 2014 9:23:49 PM
>>>>>> Subject: Re: [ovirt-devel] vdsm disabling logical volumes
>>>>>>
>>>>>> On 05/04/2014 07:57 PM, Nir Soffer wrote:
>>>>>>> ----- Original Message -----
>>>>>>>> From: "Jiri Moskovcak" <jmoskovc at redhat.com>
>>>>>>>> To: devel at ovirt.org
>>>>>>>> Sent: Sunday, May 4, 2014 8:08:33 PM
>>>>>>>> Subject: [ovirt-devel] vdsm disabling logical volumes
>>>>>>>>
>>>>>>>> Greetings vdsm developers!
>>>>>>>>
>>>>>>>> While working on adding ISCSI support to the hosted engine tools, I
>>>>>>>> ran
>>>>>>>> into a problem with vdms. It seems that when stopped vdsm deactivates
>>>>>>>> ALL logical volumes in it's volume group and when it starts it
>>>>>>>> reactivates only specific logical volumes. This is a problem for
>>>>>>>> hosted
>>>>>>>> engine tools as they create logical volumes in the same volume group
>>>>>>>> and
>>>>>>>> when vdsm deactivates the LVs the hosted engine tools don't have a way
>>>>>>>> to reactivate it, because the services drop the root permissions and
>>>>>>>> are
>>>>>>>> running as vdsm and apparently only root can activate LVs.
>>>>>>>
>>>>>>> Can you describe what volumes are you creating, and why?
>>>>>>
>>>>>> We create hosted-engine.lockspace (for sanlock) and
>>>>>> hosted-engine.metadata (keeps data about hosted engine hosts)
>>>>>
>>>>> Do you create these lvs in every vdsm vg?
>>>>
>>>> - only in the first vg created by vdsm while deploying hosted-engine
>
> It seems that the hosted engine has single point of failure - the random
> vg that contains hosted engine data.
>
>>>>
>>>>> Is this part of the domain structure
>>>>> used by hosted engine, or it has nothing to do with the storage domain?
>>>>
>>>> - sorry, I don't understand this question. How can I tell if it has
>>>> something to do with the storage domain? It's for storing data about
>>>> hosts set up to run the hosted-engine and data about state of engine and
>>>> the state of VM running the engine.
>>>
>>> Can you tell us exactly what lvs you are creating, and on which vg?
>>>
>>> And how are you creating those lvs - I guess through vdsm?
>>>
>>
>> - no hosted-engine tools do that by calling:
>>
>> lvc = popen(stdin=subprocess.PIPE, stdout=subprocess.PIPE,
>>                       stderr=subprocess.PIPE,
>>                       args=["lvm", "lvcreate", "-L", str(size_bytes)+"B",
>>                             "-n", lv_name, vg_uuid])
>> ..
>
> How do you ensure that another host is not modifying the same vg in the
> same time?
>
> If you are not ensuring this, you will corrupt this vg sooner or later.
>
> When a storage domain is detached from a host, for example when the host
> is in maintenance mode, lvs on the shared storage may be deleted, invalidating
> the devices mapper maps for these devices. If you write to an lv with wrong
> maps, you may be writing to an extent belonging to another lv, corrupting that
> lv data, or even worse corrupting the engine vg data.
>
> How do you ensure that the lvs are not deleted while you are using them?
>
>>
>>> The output of lvs command on a host with hosted engine installed will
>>> help us to understand what you are doing, and then we can think more
>>> clearly
>>> what would be the best way to support this in vdsm.
>>
>> The output of lvs: http://fpaste.org/99196/93619139/
>>
>> HE created these two LVs:
>> ha_agent-hosted-engine.lockspace
>> ha_agent-hosted-engine.metadata
>
> Why do you create these lvs on a vg owned by vdsm?
>
> If you want total control of these lvs, I suggest that you create your own
> vg and put what ever lvs you like there.
>

I would rather not go this way (at least not for 3.5) as it's too much 
code changes in hosted-engine. On the other hand the logic in vdsm seems 
wrong because it's not complementary (disabling all LVs and then 
enabling just some of them) and should be fixed anyway. This problem is 
blocking one of our 3.5 features so I've created rhbz#1094657 to track it.

--Jirka

>>
>> --Jirka
>>>
>>>>
>>>>>
>>>>>>
>>>>>>>
>>>>>>>> So far the
>>>>>>>> only suitable solution seems to be to change vdsm to only
>>>>>>>> deactivate/activate it's own LVs.
>>>>>>>
>>>>>>> This sounds reasonable. You can add a list of hosted engine lv names
>>>>>>> and skip these volumes when deactivating vdsm volumes.
>>>>>>
>>>>>> - this sounds a bit suboptimal, vdsm already has list of it's LVs, so it
>>>>>> can just disable only LVs known to it, otherwise we would have to change
>>>>>> the list everytime we add some LV to the group
>>>>>
>>>>> vdsm has a list of special lvs, that needs special treatment. Otherwise,
>>>>> it
>>>>> consider any other lv as owned by vdsm, and will deactivate them when
>>>>> they
>>>>> are
>>>>> not used.
>>>>>
>>>>> I agree that this will create a dependency, but this can also be solved.
>>>>> For example, vdsm can load the list from a file installed by hosted
>>>>> engine,
>>>>> like the typical conf.d directories.
>>>>>
>>>>
>>>> - ok, this is something I actually don't have strong opinion about, for
>>>> me adding a file with a list of LVs or tagging the logical volumes is
>>>> almost the same, I just need a way to tell vdsm which LVs to ignore..
>>>>
>>>>>>
>>>>>>>
>>>>>>> Another solution is to tag hosted engine lvs, and have vdsm ignore
>>>>>>> lvs that contains this tag.
>>>>>>
>>>>>> - this sounds good, because if we teach vdsm to ignore LVs with some tag
>>>>>> we can add new LVs in future without changing vdsm. This however applies
>>>>>> also to the solution where vdsm only disables it's own LVs,
>>>>>
>>>>> vdsm own lvs are *all* lvs in vdsm vgs. We can implement something like
>>>>> this
>>>>> using some historic tags we keep (e.g. RHAT_*), but I'd rather add new
>>>>> tag
>>>>> with
>>>>> clear semantic than use some random historic value we have.
>>>>>
>>>>>> so it
>>>>>> depends on vdsm devels which solution they find better. I think the
>>>>>> solution without tags is better, because is simpler and others (like
>>>>>> hosted-engine) can just createlv and don't bother with tags..
>>>>>
>>>>> I think that a generic tag like OVIRT_IGNORE is an easy solution for
>>>>> everyone.
>>>>>
>>>>> Federico, what do you think?
>>>>>
>>>>> Nir
>>>>>
>>>>
>>>>
>>
>>