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docs: prune: improve grid example

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Christian Schwarz 2021-07-08 19:46:24 +02:00
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docs/configuration/prune.rst
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@ -124,43 +124,51 @@ The syntax to describe the bucket list is as follows:
::
This grid spec produces the following list of adjacent buckets. For the sake of simplicity,
we subject all snapshots to the grid pruning policy by settings `regex: .*`.
Assume the following grid spec:
`
grid: 1x1h(keep=all) | 2x2h | 1x3h
regex: .*
`
grid: 1x1h(keep=all) | 2x2h | 1x3h
0h 1h 2h 3h 4h 5h 6h 7h 8h 9h
| | | | | | | | | |
|-Bucket1-|-----Bucket 2------|------Bucket 3-----|-----------Bucket 4----------|
| keep=all| keep=1 | keep=1 | keep=1 |
This grid spec produces the following constellation of buckets:
0h 1h 2h 3h 4h 5h 6h 7h 8h 9h
| | | | | | | | | |
|-Bucket1-|-----Bucket2-------|------Bucket3------|-----------Bucket4-----------|
| keep=all| keep=1 | keep=1 | keep=1 |
Let us consider the following set of snapshots @a-zA-C:
Now assume that we have a set of snapshots @a, @b, ..., @D.
Snapshot @a is the most recent snapshot.
Snapshot @D is the oldest snapshot, it is almost 9 hours older than snapshot @a.
We place the snapshots on the same timeline as the buckets:
| a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D |
0h 1h 2h 3h 4h 5h 6h 7h 8h 9h
| | | | | | | | | |
|-Bucket1-|-----Bucket2-------|------Bucket3------|-----------Bucket4-----------|
| keep=all| keep=1 | keep=1 | keep=1 |
| | | | |
| a b c| d e f g h i j k l m n o p |q r s t u v w x y z |A B C D
The `grid` algorithm maps them to their respective buckets:
The result is the following mapping of snapshots to buckets:
Bucket 1: a, b, c
Bucket 2: d,e,f,g,h,i,j
Bucket 3: k,l,m,n,o,p
Bucket 4: q,r, q,r,s,t,u,v,w,x,y,z
None: A,B,C,D
Bucket1: a, b, c
Bucket2: d,e,f,g,h,i,j
Bucket3: k,l,m,n,o,p
Bucket4: q,r,s,t,u,v,w,x,y,z
No bucket: A,B,C,D
It then applies the per-bucket pruning logic described above which resulting in the
following list of remaining snapshots.
For each bucket, we now prune snapshots until it only contains `keep` snapshots.
Newer snapshots are destroyed first.
Snapshots that do not fall into a bucket are always destroyed.
| a b c j p z |
Note that it only makes sense to grow (not shorten) the interval duration for buckets
further in the past since each bucket acts like a low-pass filter for incoming snapshots
and adding a less-low-pass-filter after a low-pass one has no effect.
Result after pruning:
0h 1h 2h 3h 4h 5h 6h 7h 8h 9h
| | | | | | | | | |
|-Bucket1-|-----Bucket2-------|------Bucket3------|-----------Bucket4-----------|
| | | | |
| a b c| j p | z |
.. _prune-keep-last-n: