docs/Cluster-VictoriaMetrics.md: update cluster scalability tips

README.md

@@ -320,11 +320,12 @@ It is recommended leaving the following amounts of spare resources:
 
 Some capacity planning tips for VictoriaMetrics cluster:
 
-- The [replication](#replication-and-data-safety) increases the amounts of needed resources for the cluster by up to `N` times where `N` is replication factor.
-- Cluster capacity for [active time series](https://docs.victoriametrics.com/FAQ.html#what-is-an-active-time-series) can be increased by adding more `vmstorage` nodes and/or by increasing RAM and CPU resources per each `vmstorage` node.
-- Query latency can be reduced by increasing the number of `vmstorage` nodes and/or by increasing RAM and CPU resources per each `vmselect` node.
-- The total number of CPU cores needed for all the `vminsert` nodes can be calculated from the ingestion rate: `CPUs = ingestion_rate / 100K`.
-- The `-rpc.disableCompression` command-line flag at `vminsert` nodes can increase ingestion capacity at the cost of higher network bandwidth usage between `vminsert` and `vmstorage`.
+- The [replication](#replication-and-data-safety) increases the amounts of needed resources for the cluster by up to `N` times where `N` is replication factor. This is because `vminsert` stores `N` copies of every ingested sample on distinct `vmstorage` nodes. These copies are de-duplicated by `vmselect` during querying. The most cost-efficient and performant solution for data durability is to rely on replicated durable persistent disks such as [Google Compute persistent disks](https://cloud.google.com/compute/docs/disks#pdspecs) instead of using the [replication at VictoriaMetrics level](#replication-and-data-safety).
+- It is recommended to run a cluster with big number of small `vmstorage` nodes instead of a cluster with small number of big `vmstorage` nodes. This increases chances that the cluster remains available and stable when some of `vmstorage` nodes are temporarily unavailable during maintenance events such as upgrades, configuration changes or migrations. For example, when a cluster contains 10 `vmstorage` nodes and a single node becomes temporarily unavailable, then the workload on the remaining 9 nodes increases by `1/9=11%`. When a cluster contains 3 `vmstorage` nodes and a single node becomes temporarily unavailable, then the workload on the remaining 2 nodes increases by `1/2=50%`. The remaining `vmstorage` nodes may have no enough free capacity for handling the increased workload. In this case the cluster may become overloaded, which may result to decreased availability and stability.
+- Cluster capacity for [active time series](https://docs.victoriametrics.com/FAQ.html#what-is-an-active-time-series) can be increased by increasing RAM and CPU resources per each `vmstorage` node or by by adding new `vmstorage` nodes.
+- Query latency can be reduced by increasing CPU resources per each `vmselect` node, since each incoming query is processed by a single `vmselect` node. Performance for heavy queries scales with the number of available CPU cores at `vmselect` node, since `vmselect` processes time series referred by the query on all the available CPU cores.
+- If the cluster needs to process incoming queries at a high rate, then its capacity can be increased by adding more `vmselect` nodes, so incoming queries could be spread among bigger number of `vmselect` nodes.
+- By default `vminsert` compresses the data sent to `vmstorage` nodes in order to reduce network bandwidth usage. The compression takes additional CPU resources. If `vminsert` nodes have limited CPU, then the compression can be disabled by passing `-rpc.disableCompression` command-line flag at `vminsert` nodes.
 
 ## High availability
 
@@ -364,30 +365,17 @@ It is available in the [helm-charts](https://github.com/VictoriaMetrics/helm-cha
 
 ## Replication and data safety
 
-By default VictoriaMetrics offloads replication to the underlying storage pointed by `-storageDataPath`.
+By default VictoriaMetrics offloads replication to the underlying storage pointed by `-storageDataPath` such as [Google compute persistent disk](https://cloud.google.com/compute/docs/disks#pdspecs), which guarantees data durability. VictoriaMetrics supports application-level replication if replicated durable persistent disks cannot be used for some reason.
 
-The replication can be enabled by passing `-replicationFactor=N` command-line flag to `vminsert`.
-This guarantees that all the data remains available for querying if up to `N-1` `vmstorage` nodes are unavailable.
-The cluster must contain at least `2*N-1` `vmstorage` nodes, where `N`
-is replication factor, in order to maintain the given replication factor for newly ingested data when `N-1` of storage nodes are lost.
-For example, when `-replicationFactor=3` is passed to `vminsert`, then it replicates all the ingested data to 3 distinct `vmstorage` nodes,
-so up to 2 `vmstorage` nodes can be lost without data loss. The minimum number of `vmstorage` nodes should be equal to `2*3-1 = 5`, so when 2 `vmstorage` nodes are lost,
-the remaining 3 `vmstorage` nodes could provide the `-replicationFactor=3` for newly ingested data.
+The replication can be enabled by passing `-replicationFactor=N` command-line flag to `vminsert`. This instructs `vminsert` to store `N` copies for every ingested sample on `N` distinct `vmstorage` nodes. This guarantees that all the stored data remains available for querying if up to `N-1` `vmstorage` nodes are unavailable.
 
-When the replication is enabled, `-dedup.minScrapeInterval=1ms` command-line flag must be passed to `vmselect` nodes.
-Optional `-replicationFactor=N` command-line flag can be passed to `vmselect` for improving query performance when up to `N-1` vmstorage nodes respond slowly and/or temporarily unavailable, since `vmselect` doesn't wait for responses from up to `N-1` `vmstorage` nodes. Sometimes `-replicationFactor` at `vmselect` nodes can result in partial responses. See [this issues](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1207) for details.
-The `-dedup.minScrapeInterval=1ms` de-duplicates replicated data during queries. If duplicate data is pushed to VictoriaMetrics from identically configured [vmagent](https://docs.victoriametrics.com/vmagent.html) instances or Prometheus instances, then the `-dedup.minScrapeInterval` must be set to bigger values according to [deduplication docs](#deduplication).
+The cluster must contain at least `2*N-1` `vmstorage` nodes, where `N` is replication factor, in order to maintain the given replication factor for newly ingested data when `N-1` of storage nodes are unavailable.
 
-Note that [replication doesn't save from disaster](https://medium.com/@valyala/speeding-up-backups-for-big-time-series-databases-533c1a927883),
-so it is recommended performing regular backups. See [these docs](#backups) for details.
+When the replication is enabled, `-dedup.minScrapeInterval=1ms` command-line flag must be passed to `vmselect` nodes, so they could de-duplicate replicated samples obtained from distinct `vmstorage` nodes during querying. If duplicate data is pushed to VictoriaMetrics from identically configured [vmagent](https://docs.victoriametrics.com/vmagent.html) instances or Prometheus instances, then the `-dedup.minScrapeInterval` must be set to bigger values according to [deduplication docs](#deduplication).
 
-Note that the replication increases resource usage - CPU, RAM, disk space, network bandwidth - by up to `-replicationFactor` times. So it may be worth
-offloading the replication to underlying storage pointed by `-storageDataPath` such as [Google Compute Engine persistent disk](https://cloud.google.com/compute/docs/disks/#pdspecs),
-which is protected from data loss and data corruption. It also provide consistently high performance
-and [may be resized](https://cloud.google.com/compute/docs/disks/add-persistent-disk) without downtime.
-HDD-based persistent disks should be enough for the majority of use cases.
+Note that [replication doesn't save from disaster](https://medium.com/@valyala/speeding-up-backups-for-big-time-series-databases-533c1a927883), so it is recommended performing regular backups. See [these docs](#backups) for details.
 
-It is recommended using durable replicated persistent volumes in Kubernetes.
+Note that the replication increases resource usage - CPU, RAM, disk space, network bandwidth - by up to `-replicationFactor=N` times, because `vminsert` stores `N` copies of incoming data to distinct `vmstorage` nodes and `vmselect` needs to de-duplicate the replicated data obtained from `vmstorage` nodes during querying. So it is more cost-effective to offload the replication to underlying replicated durable storage pointed by `-storageDataPath` such as [Google Compute Engine persistent disk](https://cloud.google.com/compute/docs/disks/#pdspecs), which is protected from data loss and data corruption. It also provide consistently high performance and [may be resized](https://cloud.google.com/compute/docs/disks/add-persistent-disk) without downtime. HDD-based persistent disks should be enough for the majority of use cases. It is recommended using durable replicated persistent volumes in Kubernetes.
 
 ## Deduplication
 

docs/Cluster-VictoriaMetrics.md

@@ -324,11 +324,12 @@ It is recommended leaving the following amounts of spare resources:
 
 Some capacity planning tips for VictoriaMetrics cluster:
 
-- The [replication](#replication-and-data-safety) increases the amounts of needed resources for the cluster by up to `N` times where `N` is replication factor.
-- Cluster capacity for [active time series](https://docs.victoriametrics.com/FAQ.html#what-is-an-active-time-series) can be increased by adding more `vmstorage` nodes and/or by increasing RAM and CPU resources per each `vmstorage` node.
-- Query latency can be reduced by increasing the number of `vmstorage` nodes and/or by increasing RAM and CPU resources per each `vmselect` node.
-- The total number of CPU cores needed for all the `vminsert` nodes can be calculated from the ingestion rate: `CPUs = ingestion_rate / 100K`.
-- The `-rpc.disableCompression` command-line flag at `vminsert` nodes can increase ingestion capacity at the cost of higher network bandwidth usage between `vminsert` and `vmstorage`.
+- The [replication](#replication-and-data-safety) increases the amounts of needed resources for the cluster by up to `N` times where `N` is replication factor. This is because `vminsert` stores `N` copies of every ingested sample on distinct `vmstorage` nodes. These copies are de-duplicated by `vmselect` during querying. The most cost-efficient and performant solution for data durability is to rely on replicated durable persistent disks such as [Google Compute persistent disks](https://cloud.google.com/compute/docs/disks#pdspecs) instead of using the [replication at VictoriaMetrics level](#replication-and-data-safety).
+- It is recommended to run a cluster with big number of small `vmstorage` nodes instead of a cluster with small number of big `vmstorage` nodes. This increases chances that the cluster remains available and stable when some of `vmstorage` nodes are temporarily unavailable during maintenance events such as upgrades, configuration changes or migrations. For example, when a cluster contains 10 `vmstorage` nodes and a single node becomes temporarily unavailable, then the workload on the remaining 9 nodes increases by `1/9=11%`. When a cluster contains 3 `vmstorage` nodes and a single node becomes temporarily unavailable, then the workload on the remaining 2 nodes increases by `1/2=50%`. The remaining `vmstorage` nodes may have no enough free capacity for handling the increased workload. In this case the cluster may become overloaded, which may result to decreased availability and stability.
+- Cluster capacity for [active time series](https://docs.victoriametrics.com/FAQ.html#what-is-an-active-time-series) can be increased by increasing RAM and CPU resources per each `vmstorage` node or by by adding new `vmstorage` nodes.
+- Query latency can be reduced by increasing CPU resources per each `vmselect` node, since each incoming query is processed by a single `vmselect` node. Performance for heavy queries scales with the number of available CPU cores at `vmselect` node, since `vmselect` processes time series referred by the query on all the available CPU cores.
+- If the cluster needs to process incoming queries at a high rate, then its capacity can be increased by adding more `vmselect` nodes, so incoming queries could be spread among bigger number of `vmselect` nodes.
+- By default `vminsert` compresses the data sent to `vmstorage` nodes in order to reduce network bandwidth usage. The compression takes additional CPU resources. If `vminsert` nodes have limited CPU, then the compression can be disabled by passing `-rpc.disableCompression` command-line flag at `vminsert` nodes.
 
 ## High availability
 
@@ -368,30 +369,17 @@ It is available in the [helm-charts](https://github.com/VictoriaMetrics/helm-cha
 
 ## Replication and data safety
 
-By default VictoriaMetrics offloads replication to the underlying storage pointed by `-storageDataPath`.
+By default VictoriaMetrics offloads replication to the underlying storage pointed by `-storageDataPath` such as [Google compute persistent disk](https://cloud.google.com/compute/docs/disks#pdspecs), which guarantees data durability. VictoriaMetrics supports application-level replication if replicated durable persistent disks cannot be used for some reason.
 
-The replication can be enabled by passing `-replicationFactor=N` command-line flag to `vminsert`.
-This guarantees that all the data remains available for querying if up to `N-1` `vmstorage` nodes are unavailable.
-The cluster must contain at least `2*N-1` `vmstorage` nodes, where `N`
-is replication factor, in order to maintain the given replication factor for newly ingested data when `N-1` of storage nodes are lost.
-For example, when `-replicationFactor=3` is passed to `vminsert`, then it replicates all the ingested data to 3 distinct `vmstorage` nodes,
-so up to 2 `vmstorage` nodes can be lost without data loss. The minimum number of `vmstorage` nodes should be equal to `2*3-1 = 5`, so when 2 `vmstorage` nodes are lost,
-the remaining 3 `vmstorage` nodes could provide the `-replicationFactor=3` for newly ingested data.
+The replication can be enabled by passing `-replicationFactor=N` command-line flag to `vminsert`. This instructs `vminsert` to store `N` copies for every ingested sample on `N` distinct `vmstorage` nodes. This guarantees that all the stored data remains available for querying if up to `N-1` `vmstorage` nodes are unavailable.
 
-When the replication is enabled, `-dedup.minScrapeInterval=1ms` command-line flag must be passed to `vmselect` nodes.
-Optional `-replicationFactor=N` command-line flag can be passed to `vmselect` for improving query performance when up to `N-1` vmstorage nodes respond slowly and/or temporarily unavailable, since `vmselect` doesn't wait for responses from up to `N-1` `vmstorage` nodes. Sometimes `-replicationFactor` at `vmselect` nodes can result in partial responses. See [this issues](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1207) for details.
-The `-dedup.minScrapeInterval=1ms` de-duplicates replicated data during queries. If duplicate data is pushed to VictoriaMetrics from identically configured [vmagent](https://docs.victoriametrics.com/vmagent.html) instances or Prometheus instances, then the `-dedup.minScrapeInterval` must be set to bigger values according to [deduplication docs](#deduplication).
+The cluster must contain at least `2*N-1` `vmstorage` nodes, where `N` is replication factor, in order to maintain the given replication factor for newly ingested data when `N-1` of storage nodes are unavailable.
 
-Note that [replication doesn't save from disaster](https://medium.com/@valyala/speeding-up-backups-for-big-time-series-databases-533c1a927883),
-so it is recommended performing regular backups. See [these docs](#backups) for details.
+When the replication is enabled, `-dedup.minScrapeInterval=1ms` command-line flag must be passed to `vmselect` nodes, so they could de-duplicate replicated samples obtained from distinct `vmstorage` nodes during querying. If duplicate data is pushed to VictoriaMetrics from identically configured [vmagent](https://docs.victoriametrics.com/vmagent.html) instances or Prometheus instances, then the `-dedup.minScrapeInterval` must be set to bigger values according to [deduplication docs](#deduplication).
 
-Note that the replication increases resource usage - CPU, RAM, disk space, network bandwidth - by up to `-replicationFactor` times. So it may be worth
-offloading the replication to underlying storage pointed by `-storageDataPath` such as [Google Compute Engine persistent disk](https://cloud.google.com/compute/docs/disks/#pdspecs),
-which is protected from data loss and data corruption. It also provide consistently high performance
-and [may be resized](https://cloud.google.com/compute/docs/disks/add-persistent-disk) without downtime.
-HDD-based persistent disks should be enough for the majority of use cases.
+Note that [replication doesn't save from disaster](https://medium.com/@valyala/speeding-up-backups-for-big-time-series-databases-533c1a927883), so it is recommended performing regular backups. See [these docs](#backups) for details.
 
-It is recommended using durable replicated persistent volumes in Kubernetes.
+Note that the replication increases resource usage - CPU, RAM, disk space, network bandwidth - by up to `-replicationFactor=N` times, because `vminsert` stores `N` copies of incoming data to distinct `vmstorage` nodes and `vmselect` needs to de-duplicate the replicated data obtained from `vmstorage` nodes during querying. So it is more cost-effective to offload the replication to underlying replicated durable storage pointed by `-storageDataPath` such as [Google Compute Engine persistent disk](https://cloud.google.com/compute/docs/disks/#pdspecs), which is protected from data loss and data corruption. It also provide consistently high performance and [may be resized](https://cloud.google.com/compute/docs/disks/add-persistent-disk) without downtime. HDD-based persistent disks should be enough for the majority of use cases. It is recommended using durable replicated persistent volumes in Kubernetes.
 
 ## Deduplication