[ClusterLabs] Antw: Re: Antw: [EXT] unexpected fenced node and promotion of the new master PAF ‑ postgres

[Ulrich Windl]

>>> Jehan-Guillaume de Rorthais <jgdr at dalibo.com> schrieb am 09.10.2021 um
21:28 in
Nachricht <20211009212843.1c8469af at firost>:

...
> Indeed. But it's an arbitrage between swapping process mem or freeing
> mem by removing data from cache. For database servers, it is advised to use

> a
> lower value for swappiness anyway, around 5‑10, as a swapped process means
> longer query, longer data in caches, piling sessions, etc.

Processes are not swapped, but _pages_ are paged out. That means that
typically uused code paths and data may be paged out.
Database systems may consist of megabytes of error message texts that are not
actually out put in years.
The interesting number is not the "swap" proportion, but the swap (i.e.
paging) _rate_.

> 
> But I still doubt corosync could be swapped, unless it cried on startup it
> couldn't mlock its memory.
> 
>> What is your kernel version? On several consecutive kernel versions I
>> observed the following effect ‑ once swap started being used at all
>> system experienced periodical stalls for several seconds. It feeled like
>> frozen system. It did not matter how much swap was in allocated ‑
>> several megabytes was already enough.
>> 
>> As far as I understand, the problem was not really time to swap out/in,
>> but time kernel spent traversing page tables to make decision. I think
>> it start with kernel 5.3 (or may be 5.2) and I do not see it any more
>> since I believe kernel 5.7.
> 
> Interesting.
> 
> ++
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