EgorBaranovEnjoysTyping commented on code in PR #13714:
URL: https://github.com/apache/ignite/pull/13714#discussion_r4358907342
##########
docs/_docs/data-rebalancing.adoc:
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@@ -28,6 +28,83 @@ In pure in-memory clusters, the default behavior is to start
rebalancing immedia
In clusters with persistence, the baseline topology has to be changed manually
(default behavior), or can be changed automatically when
link:clustering/baseline-topology#baseline-topology-autoadjustment[automatic
baseline adjustment] is enabled.
====
+== Rebalance Strategy [[rebalance-strategy]]
+
+When a node joins or rejoins the cluster, each partition is restored using one
of two strategies:
+
+- *Full rebalance* — the node downloads the entire partition content from
another server node that holds a copy of the partition.
+- *Historical rebalance* — the node restores its own data from the local disk
and downloads only the delta (WAL entries written while it was absent) from the
source node that holds a copy of the partition.
+
+The strategy is selected per partition based on the following rules:
+
+. A full rebalance is used if the node has no local data for the partition
(update counter is zero).
++
+This applies both to a *new node joining for the first time* (it has no data
on disk at all) and to a node that was *evicted from the baseline* and rejoined
(its local partition data was invalidated). In both cases the full partition
content must be downloaded from another node.
+
+. A full rebalance is used if the delta exceeds the total number of entries in
the partition — transferring the full content is more efficient in that case.
++
+Example: a partition contains 100 entries. While the node was absent, 500
writes hit this partition. Downloading 500 delta entries is more expensive than
just downloading all 100 entries → full rebalance.
+
+. A full rebalance is used if the partition size is below the
`historical.rebalance.threshold` (DMS property, default 500 entries), unless
the `IGNITE_PREFER_WAL_REBALANCE` system property is set to `true`.
Review Comment:
It's the standard abbreviation used throughout the codebase
(DistributedMetaStorage,
GridDistributedMetastorageManager). Expanding it to "Distributed
MetaStorage" (two words) would actually be incorrect — the canonical name is
"Distributed Meta Storage" (three words).
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