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docs/Single-server-VictoriaMetrics.md: explain why recently inserted data may be unavailable for querying for a few seconds

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Aliaksandr Valialkin 2020-11-13 18:33:53 +02:00
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docs/Single-server-VictoriaMetrics.md
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@ -1209,6 +1209,12 @@ VictoriaMetrics also exposes currently running queries with their execution time
* It is recommended inspecting logs during troubleshooting, since they may contain useful information. * It is recommended inspecting logs during troubleshooting, since they may contain useful information.
* VictoriaMetrics buffers incoming data in memory for up to a few seconds before flushing it to persistent storage.
This may lead to the following "issues":
* Data becomes available for querying in a few seconds after inserting.
* The last few seconds of inserted data may be lost on unclean shutdown (i.e. OOM, `kill -9` or hardware reset).
See [this article for technical details](https://valyala.medium.com/wal-usage-looks-broken-in-modern-time-series-databases-b62a627ab704).
* If VictoriaMetrics works slowly and eats more than a CPU core per 100K ingested data points per second, * If VictoriaMetrics works slowly and eats more than a CPU core per 100K ingested data points per second,
then it is likely you have too many active time series for the current amount of RAM. then it is likely you have too many active time series for the current amount of RAM.
VictoriaMetrics [exposes](#monitoring) `vm_slow_*` metrics, which could be used as an indicator of low amounts of RAM. VictoriaMetrics [exposes](#monitoring) `vm_slow_*` metrics, which could be used as an indicator of low amounts of RAM.