[ovirt-devel] Memory stats in VDSM

[Juan Hernández]

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