docs/cluster: update cluster resizing info (#6099)

* docs/cluster: update cluster resizing info

- add example of resources distribution
- add info about how to handle uneven disk usage after adding a new storage node

Signed-off-by: Zakhar Bessarab <z.bessarab@victoriametrics.com>

* Update docs/Cluster-VictoriaMetrics.md

Co-authored-by: Roman Khavronenko <roman@victoriametrics.com>

* Update docs/Cluster-VictoriaMetrics.md

Co-authored-by: Roman Khavronenko <roman@victoriametrics.com>

---------

Signed-off-by: Zakhar Bessarab <z.bessarab@victoriametrics.com>
Co-authored-by: Roman Khavronenko <roman@victoriametrics.com>

docs/Cluster-VictoriaMetrics.md

@@ -459,8 +459,8 @@ Cluster performance and capacity can be scaled up in two ways:
 General recommendations for cluster scalability:
 
 - Adding more CPU and RAM to existing `vmselect` nodes improves the performance for heavy queries, which process big number of time series with big number of raw samples. See [this article on how to detect and optimize heavy queries](https://valyala.medium.com/how-to-optimize-promql-and-metricsql-queries-85a1b75bf986).
-- Adding more `vmstorage` nodes increases the number of [active time series](https://docs.victoriametrics.com/FAQ.html#what-is-an-active-time-series) the cluster can handle. This also increases query performance over time series with [high churn rate](https://docs.victoriametrics.com/FAQ.html#what-is-high-churn-rate). The cluster stability is also improved with the number of `vmstorage` nodes, since active `vmstorage` nodes need to handle lower additional workload when some of `vmstorage` nodes become unavailable.
-- Adding more CPU and RAM to existing `vmstorage` nodes increases the number of [active time series](https://docs.victoriametrics.com/FAQ.html#what-is-an-active-time-series) the cluster can handle. It is preferred to add more `vmstorage` nodes over adding more CPU and RAM to existing `vmstorage` nodes, since higher number of `vmstorage` nodes increases cluster stability and improves query performance over time series with [high churn rate](https://docs.victoriametrics.com/FAQ.html#what-is-high-churn-rate).
+- Adding more `vmstorage` nodes (aka horizontal scaling) increases the number of [active time series](https://docs.victoriametrics.com/FAQ.html#what-is-an-active-time-series) the cluster can handle. This also increases query performance over time series with [high churn rate](https://docs.victoriametrics.com/FAQ.html#what-is-high-churn-rate). The cluster stability is also improved with the number of `vmstorage` nodes, since active `vmstorage` nodes need to handle lower additional workload when some of `vmstorage` nodes become unavailable. For example, during a node outage the rest of the nodes will pick up the load designated to unavailable node. So in case one node out of 5 is unavailable 20% of the load will be re-distributed across 4 remaining nodes which means each node will take 5% of the load. With 10 nodes in case of outage 10% of the load will be distributed across 9 remaining node, so around ~1% of load will be distributed across other nodes.
+- Adding more CPU and RAM to existing `vmstorage` nodes (aka vertical scaling) increases the number of [active time series](https://docs.victoriametrics.com/FAQ.html#what-is-an-active-time-series) the cluster can handle. It is preferred to add more `vmstorage` nodes over adding more CPU and RAM to existing `vmstorage` nodes, since higher number of `vmstorage` nodes increases cluster stability and improves query performance over time series with [high churn rate](https://docs.victoriametrics.com/FAQ.html#what-is-high-churn-rate).
 - Adding more `vminsert` nodes increases the maximum possible data ingestion speed, since the ingested data may be split among bigger number of `vminsert` nodes.
 - Adding more `vmselect` nodes increases the maximum possible queries rate, since the incoming concurrent requests may be split among bigger number of `vmselect` nodes.
 
@@ -470,6 +470,8 @@ Steps to add `vmstorage` node:
 1. Gradually restart all the `vmselect` nodes with new `-storageNode` arg containing `<new_vmstorage_host>`.
 1. Gradually restart all the `vminsert` nodes with new `-storageNode` arg containing `<new_vmstorage_host>`.
 
+In order to handle uneven disk space usage distribution after adding new `vmstorage` node it is possible to update `vminsert` configuration to route newly ingested metrics only to new storage nodes. Once disk usage will be similar configuration can be updated to include all nodes again. Note that `vmselect` nodes need to reference all storage nodes for querying.
+
 ## Updating / reconfiguring cluster nodes
 
 All the node types - `vminsert`, `vmselect` and `vmstorage` - may be updated via graceful shutdown.