分片分配与再平衡
了解 Elasticsearch 如何决定分片的存放位置、分配感知如何将副本分散到不同硬件上，以及如何在扩容和维护期间控制再平衡。
分片分配与再平衡 是 CoddyKit 上的免费 Elasticsearch & Full Text Search Systems 课时。 这是第 4 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 Elasticsearch & Full Text Search Systems 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Elasticsearch & Full Text Search Systems 课程共包含 4 节课。
本课时的部分内容尚未翻译，以英文显示。
Where Do Shards Go?
When you create an index or add a node, Elasticsearch must decide which node holds each shard. This process is shard allocation, and keeping it balanced is key to a healthy, performant cluster.
The Allocation Process
The master node runs allocation deciders for every unassigned shard. They check disk space, shard count, awareness rules, and filters before placing a shard. Failing any decider keeps the shard UNASSIGNED.
Primary vs Replica Placement
A replica is never allocated to the same node as its primary. This guarantees that losing one node cannot take out both copies of the same data. With one replica you survive any single-node failure.
Watching Allocation
Use the _cat/shards API to see where each shard sits and its state. STARTED means active; RELOCATING means it is moving.
GET _cat/shards?vExplaining a Stuck Shard
When a shard will not allocate, the _cluster/allocation/explain API tells you exactly which decider blocked it and why. This is the first stop for debugging a yellow or red cluster.
GET _cluster/allocation/explain
{
"index": "my_index",
"shard": 0,
"primary": false
}Allocation Awareness
Allocation awareness spreads copies across physical boundaries like racks or availability zones. You tag nodes with custom attributes and tell the cluster to be aware of them.
cluster.routing.allocation.awareness.attributes: zoneForced Awareness
Forced awareness goes further: it refuses to over-allocate replicas to surviving zones if a zone goes down, preventing one zone from holding all copies. This protects capacity planning during outages.
cluster.routing.allocation.awareness.force.zone.values: zone1,zone2Shard Filtering
You can pin or exclude an index from specific nodes using allocation filters. This is handy for hot/warm architectures where older indices migrate to cheaper hardware.
PUT my_index/_settings
{
"index.routing.allocation.require.box_type": "warm"
}Rebalancing
After nodes join or leave, the cluster rebalances to even out shard counts. This is automatic but generates network and disk I/O, so it should be controlled during sensitive operations.
Pausing Allocation
Before a rolling restart, disable allocation so the cluster does not waste effort relocating shards from a node you are about to bring back. Re-enable it afterward.
PUT _cluster/settings
{
"transient": {
"cluster.routing.allocation.enable": "primaries"
}
}Throttling
Limit concurrent recoveries with cluster.routing.allocation.node_concurrent_recoveries and bandwidth with indices.recovery.max_bytes_per_sec to keep rebalancing from starving live queries.
Quick Check
Test your understanding of shard allocation.
Recap
You learned how shards are placed and balanced:
- Allocation deciders choose shard locations; replicas avoid their primary's node.
- Debug stuck shards with
allocation/explain. - Allocation awareness and forced awareness spread copies across zones.
- Use allocation filters for hot/warm tiers and disable allocation during rolling restarts.
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常见问题解答
「分片分配与再平衡」课时是免费的吗？
是的 — 「分片分配与再平衡」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Elasticsearch & Full Text Search Systems 课程的其余内容，请升级到 CoddyKit PRO。 Elasticsearch & Full Text Search Systems 课程共包含 4 节课。
「分片分配与再平衡」这节课中我会学到什么？
了解 Elasticsearch 如何决定分片的存放位置、分配感知如何将副本分散到不同硬件上，以及如何在扩容和维护期间控制再平衡。 你通过在浏览器中直接运行的动手代码来练习 Elasticsearch & Full Text Search Systems，全天候 AI 导师会在你学习这节课的过程中回答你的问题。
学习 Elasticsearch & Full Text Search Systems 需要有经验吗？
无需任何先前经验。CoddyKit 上的 Elasticsearch & Full Text Search Systems 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 4 节课，共 4 节。
「分片分配与再平衡」课时需要多长时间？
大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。
我能在这节 Elasticsearch & Full Text Search Systems 课中编写并运行代码吗？
能。每节 Elasticsearch & Full Text Search Systems 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。