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docs/Cluster-VictoriaMetrics.md: clarify replication docs

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Aliaksandr Valialkin 2021-02-15 01:44:11 +02:00
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docs/Cluster-VictoriaMetrics.md
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@ -338,9 +338,15 @@ It is available in the [helm-charts](https://github.com/VictoriaMetrics/helm-cha
## Replication and data safety
In order to enable application-level replication, `-replicationFactor=N` command-line flag must be passed to `vminsert`.
By default VictoriaMetrics offloads replication to the underlying storage pointed by `-storageDataPath`.
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.
For example, when `-replicationFactor=3` is passed to `vminsert`, then it replicates all the ingested data to 3 distinct `vmstorage` nodes.
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.
When the replication is enabled, `-replicationFactor=N` and `-dedup.minScrapeInterval=1ms` command-line flag must be passed to `vmselect` nodes.
The `-replicationFactor=N` improves query performance when a part of vmstorage nodes respond slowly and/or temporarily unavailable.
@ -350,9 +356,9 @@ when [deduplication](https://victoriametrics.github.io/Single-server-VictoriaMet
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.
By default VictoriaMetrics offloads replication to the underlying storage pointed by `-storageDataPath`.
It is recommended storing data on [Google Compute Engine persistent disks](https://cloud.google.com/compute/docs/disks/#pdspecs),
since they are protected from data loss and data corruption. They also provide consistently high performance
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.