Thank you both for your input! It's been a honor for me to get a response from both of you.
I would like to get an overall picture, so what I'm searching for is something like this:
We are trying to figure out how many page reads/writes (in terms of logical and physical reads/writes) are necessary to update a single document. I was trying to understand the logical and physical reads/writes by the WiredTiger Cache Stats (with mongoDb collStats/serverStatus) but I'm having a hard time to understand how that metrics works. The mongodb documentation says that if you want to have an overview of the I/O Operations you should have a look at
pages read into cache (I would understand that as physical reads?)
pages written from cache (This is my understanding for physical writes?)
I drafted a simulation with 5M entities. In the simulation, a new property is added to each document (average object size was 98 before adding the property, 117 afterwards).
But when I'm restarting mongodb (heating up the cache) and execute the simulation the metrics will read as:
"wiredTiger.cache.pages read into cache": 4452,
"wiredTiger.cache.pages written from cache": 7117,
"wiredTiger.cache.pages requested from the cache": 20165168
How is it possible that there are many more pages requested from the cache (logical reads?) than initially have been read into cache?
Sorry for my bad english - I'm a german native speaker...