[ovirt-devel] Memory stats in VDSM

[Michal Skrivanek]

> On 28 Aug 2017, at 14:02, Juan Hernández <jhernand at redhat.com> wrote:
> 
> On 08/28/2017 01:57 PM, Juan Hernández wrote:
>> On 08/28/2017 01:42 PM, Michal Skrivanek wrote:
>>> 
>>>> On 28 Aug 2017, at 13:37, Juan Hernández <jhernand at redhat.com> wrote:
>>>> 
>>>> On 08/28/2017 01:11 PM, Tomáš Golembiovský wrote:
>>>>> Hi,
>>>>> there is ongoing effort to change how VDSM collects information about
>>>>> memory usage from guests. We used to use oVirt guest agent to get the
>>>>> statistics about free memory, swap usage, etc. This is going to change
>>>>> and we will use stats provided by VirtIO Balloon driver. [1] This should
>>>>> not break MOM nor Engine (current or previous versions).
>>>>> There is however small downside to this change. We will not be able to
>>>>> fetch information about buffers and caches on Linux guests anymore
>>>>> (mem_cached and mem_buffers in memoryStats). The corollary of this is
>>>>> that free memory reported by Engine will also no longer include those
>>>>> statistics and reported free memory will be really (only) free memory.
>>>>> I now there will be mixed feelings about this and we would like to get
>>>>> the information about buffers and caches back to the VDSM. Correctly via
>>>>> balloon driver statistics. [2] This effort will however take some time.
>>>>>     Tomas
>>>>> [1] https://gerrit.ovirt.org/#/q/topic:memory-stats
>>>>> [2] https://lists.nongnu.org/archive/html/qemu-devel/2017-08/msg05239.html
>>>> 
>>>> Note that this information is currently reported by the engine REST API, so it is part of its contract. Removing it breaks backwards compatibility and causes a regression in this RFE:
>>> 
>>> we can either declare it deprecated or temporarily report 0 until the support propagates into qemu-ga
>>> 
>> So there will be an interval of time where it will not be reported? Will that affect released versions of the system or it is just something that we will re-add before the next release? It is released versions of the system that I am concerned about. We should not do a release that breaks backwards compatibility. It is completely OK if we break it in between releases.
>>> I think best way forward is to just make it optionally available when ovirt-guest-agent reports it.
>>> I.e. users will get only part of the information when they run only qemu-ga, and that’s fine…
>>> 
> 
> Not sure I understand this. Currently, if I understand correctly, these values are only populated if the ovirt-guest-agent is installed? That is OK (may be worth documenting it). We plan to replace ovirt-guest-agent with qemu-ga in a mandatory way? Or will the user be able to use one or the other?

both are mandatory today, though not really enforced, there’s just a “!” next to the VM in the VMs grid.
The plan is to switch data retrieval to qemu-ga and make ovirt-ga optional (it’s going to be mandatory for specific purposes, like SSO)

> If the user decides to use the ovirt-guest-agent, will we continue to report these statistics?

yes, I expect so

Thanks,
michal

> 
>>>> 
>>>>  [RFE] Report guests Buffered/Cached memory
>>>>  https://bugzilla.redhat.com/1024010
>