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new 7cf9f655b09 HDDS-13575. Update Erasure Coding section in the Topology
Awareness user doc (#8942)
7cf9f655b09 is described below
commit 7cf9f655b090d70f5ad680d34955f2ed8eb50c02
Author: Wei-Chiu Chuang <[email protected]>
AuthorDate: Fri Sep 19 13:18:09 2025 -0700
HDDS-13575. Update Erasure Coding section in the Topology Awareness user
doc (#8942)
---
hadoop-hdds/docs/content/feature/Topology.md | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
diff --git a/hadoop-hdds/docs/content/feature/Topology.md
b/hadoop-hdds/docs/content/feature/Topology.md
index 25db4520816..7f0e27b0bde 100644
--- a/hadoop-hdds/docs/content/feature/Topology.md
+++ b/hadoop-hdds/docs/content/feature/Topology.md
@@ -157,6 +157,24 @@ This is configured using the
`ozone.scm.container.placement.impl` property in `o
Note: When configuring these values, include the full class name prefix: for
example,
org.apache.hadoop.hdds.scm.container.placement.algorithms.SCMContainerPlacementCapacity
+## Container Placement for Erasure Coded (EC) Containers
+
+For Erasure Coded (EC) containers, SCM employs a specialized placement policy
to ensure data resilience and availability by distributing data and parity
blocks across multiple racks. This is configured using the
`ozone.scm.container.placement.ec.impl.key` property in `ozone-site.xml`.
+
+### 1. `SCMContainerPlacementRackScatter` (Default)
+
+* **Function:** This is the default policy for EC containers. It attempts to
place each block (both data and parity) of an EC container on a different rack.
For example, for an RS-6-3-1024k container (6 data blocks + 3 parity blocks),
this policy will try to place the 9 blocks on 9 different racks. This "scatter"
approach maximizes the fault tolerance, as the loss of a single rack will not
impact more than one block of the container. [5]
+* **Use Cases:** This policy is highly recommended for production clusters
using Erasure Coding to protect against rack-level failures.
+* **Configuration:**
+ ```xml
+ <property>
+ <name>ozone.scm.container.placement.ec.impl.key</name>
+
<value>org.apache.hadoop.hdds.scm.container.placement.algorithms.SCMContainerPlacementRackScatter</value>
+ </property>
+ ```
+* **Behavior:** If the number of available racks is less than the number of
blocks in the EC group, the policy will start placing more than one block on
the same rack, while trying to keep the distribution as even as possible.
+* **Limitations:** Similar to `SCMContainerPlacementRackAware`, this policy
is designed for single-layer rack topologies (e.g., `/rack/node`) and is not
recommended for multi-layer hierarchies.
+
## Optimizing Read Paths
Enable by setting `ozone.network.topology.aware.read` to `true` in
`ozone-site.xml`. [1]
@@ -173,6 +191,8 @@ This directs clients (replicated data) to read from
topologically closest DataNo
* **Accurate Topology:** Maintain an accurate, up-to-date topology map
(static or dynamic script); this is foundational.
* **Pipeline Creation:** For production environments, use the default
`PipelinePlacementPolicy` for `ozone.scm.pipeline.placement.impl` to ensure
both rack fault tolerance and pipeline load balancing.
* **Pipeline Selection:** The default `RandomPipelineChoosePolicy` for
`hdds.scm.pipeline.choose.policy.impl` is suitable for general load balancing.
+* **Replicated (RATIS) Containers:** For production, use
`SCMContainerPlacementRackAware` (mindful of its single-layer topology
limitation) or `SCMContainerPlacementCapacity` (balanced disk usage) over
`SCMContainerPlacementRandom`.
+* **Erasure Coded (EC) Containers:** For production rack fault tolerance,
use `SCMContainerPlacementRackScatter`.
* **Read Operations:** Enable `ozone.network.topology.aware.read` with
accurate topology.
* **Monitor & Validate:** Regularly monitor placement and balance; use tools
like Recon to verify topology awareness.
@@ -182,3 +202,4 @@ This directs clients (replicated data) to read from
topologically closest DataNo
2. [Ozone Source Code: container placement
policies](https://github.com/apache/ozone/tree/master/hadoop-hdds/server-scm/src/main/java/org/apache/hadoop/hdds/scm/container/placement/algorithms).
(For implementations of pluggable placement policies).
3. [Ozone Source Code:
SCMContainerPlacementRandom.java](https://github.com/apache/ozone/blob/master/hadoop-hdds/server-scm/src/main/java/org/apache/hadoop/hdds/scm/container/placement/algorithms/SCMContainerPlacementRandom.java).
4. [Ozone Source Code:
SCMContainerPlacementCapacity.java](https://github.com/apache/ozone/blob/master/hadoop-hdds/server-scm/src/main/java/org/apache/hadoop/hdds/scm/container/placement/algorithms/SCMContainerPlacementCapacity.java).
+5. [Ozone Source Code:
SCMContainerPlacementRackScatter.java](https://github.com/apache/ozone/blob/master/hadoop-hdds/server-scm/src/main/java/org/apache/hadoop/hdds/scm/container/placement/algorithms/SCMContainerPlacementRackScatter.java).
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