nickva commented on PR #5625:
URL: https://github.com/apache/couchdb/pull/5625#issuecomment-3202798994
I pretty-fied a quick btree stats reporter I had since we were wondering
what the tree looked like from above (this is q=8, 100k docs) just one shard
copy:
```json
"sizes": {
"active": 5812423,
"external": 4040930,
"file": 7188696,
"id_tree": {
"1": {
"kp_node": {
"cnt": 1,
"max": 5,
"min": 5
}
},
"2": {
"kp_node": {
"cnt": 5,
"max": 21,
"min": 15
}
},
"3": {
"kp_node": {
"cnt": 91,
"max": 23,
"min": 11
}
},
"4": {
"kv_node": {
"cnt": 1450,
"max": 15,
"min": 1
}
}
},
"seq_tree": {
"1": {
"kp_node": {
"cnt": 1,
"max": 2,
"min": 2
}
},
"2": {
"kp_node": {
"cnt": 2,
"max": 26,
"min": 2
}
},
"3": {
"kp_node": {
"cnt": 28,
"max": 47,
"min": 6
}
},
"4": {
"kv_node": {
"cnt": 975,
"max": 15,
"min": 2
}
}
}
}
```
The key is depth, then node type, then `cnt` is the number of nodes at that
level, `min` is the smallest node size (number of kvs/kps), `max` is the
largest size.
It's not as shallow as we'd expect due to how complete_root works, and
chunk_size is probably not the best any longer (doesn't count for compression).
I was going to look into maybe having a different chunk size, or different per
node type (kps get more), but that's for a different PR.
So caching the top 2 nodes makes sense, there are not that many and they
bring the biggest benefit. Top 3 could also be an option but maybe start
smaller at first.
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