[3pt] More details needed for slab_info

More details needed for slab_info

Product: Tarantool
Audience/target: who will read it and for what task?
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_slab/slab_info/
SME: @ expert to ask for more information
Peer reviewer: @ documentarian name

Details

The description of parameters and their dependency on each other is not so clear. Most understandable is quota_used_ratio - it's overall available memtx_memory utilization. items_used_ratio and arena_used_ratio is not so clear. Should I worry about them only if quota_used_ratio becomes > 0.9? What if items_used_ratio and arena_used_ratio are > 0.9, but quota_used_ratio id ~0.5? It's normal memory utilization? And if quota_used_ratio > 0.9 but items_used_ratio and arena_used_ratio < 0.5? Instructions in case of memory fragmentation would be also useful. Also in cases ratio is specified in absolute values [0..1], and in box.slab.info() output it's in percents, which may add to confusion with documentation. In example arena_used_ratio quite larger than items_used_ratio. Is it normal situation? Several different examples with analysis would be useful.

https://app.mopinion.com/dashboard/inbox/detail/4386478

Definition of done

Start with the root document at reference/reference_lua/box_slab/slab_info/ and compare its parameter descriptions with the box.slab.info() output mentioned in the issue. Clarify the relationships and units for quota_used_ratio, items_used_ratio, and arena_used_ratio, then document fragmentation guidance and add analyzed examples once the SME supplies the missing details.