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docs: compact: add blurb about how retention policy works #2808

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Jun 25, 2020
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docs: compact: add blurb about how retention policy works #2808

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2 changes: 2 additions & 0 deletions docs/components/compact.md
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Expand Up @@ -49,6 +49,8 @@ Ideally, you will have equal retention set (or no retention at all) to all resol

Not setting this flag, or setting it to `0d`, i.e. `--retention.resolution-X=0d`, will mean that samples at the `X` resolution level will be kept forever.

Please note that blocks are only deleted after they completely "fall off" of the specified retention policy. In other words, the "max time" of a block needs to be older than the amount of time you had specified.

## Storage space consumption

In fact, downsampling doesn't save you any space but instead it adds 2 more blocks for each raw block which are only slightly smaller or relatively similar size to raw block. This is required by internal downsampling implementation which to be mathematically correct holds various aggregations. This means that downsampling can increase the size of your storage a bit (~3x), but it gives massive advantage on querying long ranges.
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