I have an item that has vm.memory.size[used] as a key, this returns the memory used, this also included the cached and the buffers.
I need to subtract vm.memory.size[cached] and vm.memory.size[buffers] from the vm.memory.size[used] to get the value that I need.
How can I do this please since I cannot find a way to do this, this what I tried lately but deos not work.
Answer
If you want to calculate it in a separate item, you must have used, cached and buffers already monitored as normal items. Once you have them, the calculated item formula would be last(vm.memory.size[used])-last(vm.memory.size[cached])-last(vm.memory.size[[buffers]) .
You can also calculate that directly in a trigger, removing the need for the calculated item.
And maybe even simpler than that - vm.memory.size[available] and vm.memory.size[pavailable] item keys can give you the (raw and percentage, respectively) amount of the available memory - already excluding cache & buffers - that you might be able to alert on directly.