[ovirt-devel] Memory stats in VDSM

[Juan Hernández]

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

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