Add practical tips for downsampling.

manage-data/data-store/data-streams/run-downsampling.md

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+## Practical tips
+
+Downsampling requires reading and indexing the contents of a backing index. The following guidelines can help you get the most out of it.
+
+### Choosing the downsampling interval
+
+When choosing the downsampling interval, you need to consider the original sampling rate of your measurements. Ideally, you would like an interval that would reduce your number of documents by a significant amount. For example, if a sensor sends data every 10 seconds downsampling to 1 minute would reduce the number of documents by 83%, compared to downsampling to 5 minutes by 96%.
+
+The same applied when downsampling already downsampled data. 
+
+### Downsampling with Index Lifecycle Management
+
+The following tips apply to data streams downsampled by index lifecycle management (ILM).
+
+#### Phases and tiers
+
+When using ILM, you can define at most one downsampling round in the following phases:
+
+- `hot` phase: it will execute the downsampling after the [index time series end time](elasticsearch://reference/elasticsearch/index-settings/time-series.md#index-time-series-end-time) has passed
+- `warm` phase: it will execute the downsampling `min_age` time after the rollover (respecting the [index time series end time](elasticsearch://reference/elasticsearch/index-settings/time-series.md#index-time-series-end-time))
+- `cold` phase: it will execute the downsampling `min_age` time after the rollover (respecting the [index time series end time](elasticsearch://reference/elasticsearch/index-settings/time-series.md#index-time-series-end-time))
+
+The phases do not require the respective tiers to exist. However, when a cluster has tiers, ILM automatically migrates the data processed in the phase to the respective tier. This can be disabled by adding the [migrate action](elasticsearch://reference/elasticsearch/index-lifecycle-actions/ilm-migrate.md#ilm-migrate-options) with `enabled: false`.
+
+The migrate action is implicitly enabled, moving downsampling data to the respective tier. The downsampling operation occurs at the same tier as the source index and then the downsampled data gets migrated, allowing downsampling to leverage the resources of the "hotter" tier and move less data to the next tier.
+
+#### Reducing index size
+
+When configuring an ILM policy with downsampling, it is necessary to define the [rollover action](elasticsearch://reference/elasticsearch/index-lifecycle-actions/ilm-rollover.md) in the  `hot` phase. The rollover action consists of the conditions that would trigger a rollover hence it determines the size of an index and its shards. The size of an index can influence the impact that downsampling has on a cluster's performance. 
+
+The downsampling operation runs over a whole index, so in certain cases downsampling can increase the load on a cluster. One of the ways to reduce that load is to reduce the size of the index; this way you can have smaller downsampling tasks that get better distributed. You can achieve that either by reducing the number of primary shards or by using setting [`max_primary_shard_docs`](https://www.elastic.co/docs/reference/elasticsearch/index-lifecycle-actions/ilm-rollover#ilm-rollover-options) to reduce the number of docs in a single shard. Using a lower value than the default of 200 million is expected to help smoothen load spikes due to downsampling.
+
 ## Additional resources
 
 * [](downsampling-concepts.md)