[ovs-discuss] OVN Database sizes - Auto compact feature

[Daniel Alvarez Sanchez]

Right, thanks Mark! Good point about the 4x, I missed that one.
I'm repeating the test for 1K ports. Not sure if I'll be able to reproduce
the 2.5GB part but it is still weird that while deleting ports (actually
deleting stuff from the DB) it doesn't get to compact the DB further
(last time it shrinked to 9MB) so maybe we have something odd here
going in the online compact. I'll post the results after this test.

On Wed, Mar 7, 2018 at 3:35 PM, Mark Michelson <mmichels at redhat.com> wrote:

> On 03/07/2018 07:40 AM, Daniel Alvarez Sanchez wrote:
>
>> Hi folks,
>>
>> During the performance tests I've been doing lately I noticed
>> that the size of the Southbound database was around 2.5GB
>> in one of my setups. I couldn't dig further then but now I
>> decided to explore a bit more and these are the results in
>> my all-in-one OpenStack setup using OVN as a backend:
>>
>> * Created 800 ports on the same network (logical switch).
>> * Deleted those 800 ports.
>> * I logged the DB sizes for both NB and SB databases every second.
>>
>> See attached image for the results.
>>
>> At around x=2000, the creation task finished and deletion starts.
>> As you can see, there's automatic compact happening in the
>> NB database across the whole test. However, while I was deleting
>> ports, the SB database stop shrinking and keeps growing.
>>
>> After the test finished, the DB sizes remaining the same
>> thus SB database being around 34MB. It was not until I
>> manually compacted it when it finally shrinked:
>>
>>
>> [stack at ovn ovs]$ ls -alh ovnsb_db.db
>> -rw-r--r--. 1 stack stack 34M Mar  7 12:04 ovnsb_db.db
>>
>> [stack at ovn ovs]$ sudo ovs-appctl -t /usr/local/var/run/openvswitch/ovnsb_db.ctl
>> ovsdb-server/compact
>>
>> [stack at ovn ovs]$ ls -alh ovnsb_db.db
>> -rw-r--r--. 1 stack stack 207K Mar  7 13:32 ovnsb_db.db
>>
>> I'll try to investigate further in the code.
>> Thanks,
>>
>> Daniel
>>
>
> Daniel and I discussed this offline and I think the attached result does
> not necessarily indicate a bug yet.
>
> One of the requirements for the DB to compact automatically is that it
> needs to grow to at least 4x the size of the previous snapshot. In the
> graph, we can see that the first time the SB DB compacts (around x==1000),
> it shrinks down to ~5MB. We would expect the DB to compact again when it
> gets to ~20MB. This happens at around x==1900. The DB shrinks to ~9MB, so
> we would expect it to shrink again when it reaches ~36MB. When the test
> ends, the SB DB is ~34 MB, so it is not quite large enough to compact yet.
> If the test had gone a bit longer, then presumably we might have seen the
> DB shrink again.
>
> It will be interesting to see the test that leads to a 2.5GB southbound
> database.
>
> Mark!
>
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