HDDS-14199. [Docs] Explain how EC write pipelines are calculated (#9520)

Author: jojochuang

hadoop-hdds/docs/content/feature/ErasureCoding.md

@@ -228,6 +228,41 @@ When using ofs/o3fs, we can pass the EC Replication Config by setting the config
 
 In the case bucket already has default EC Replication Config, there is no need of passing EC Replication Config while creating key.
 
+#### Calculating EC Pipeline Limits
+
+The target number of open EC pipelines SCM aims to maintain is calculated dynamically for each EC replication configuration (e.g., RS-6-3, RS-3-2). The calculation is based on the following two properties, with the final target being the greater of the two resulting values.
+
+*   `ozone.scm.ec.pipeline.minimum`
+    *   **Description**: The guaranteed minimum number of open pipelines to maintain for each EC configuration, regardless of other factors.
+    *   **Default Value**: `5`
+
+*   `ozone.scm.ec.pipeline.per.volume.factor`
+    *   **Description**: A factor used to calculate a target number of pipelines based on the total number of healthy volumes across all datanodes in the cluster.
+    *   **Default Value**: `1.0`
+
+**Calculation Logic:**
+
+SCM first calculates a volume-based target using the formula:
+`(<pipeline.per.volume.factor> * <total healthy volumes>) / <required nodes for EC config>`
+
+The final target number of pipelines is then determined by:
+`max(<volume-based target>, <pipeline.minimum>)`
+
+**Example:**
+
+Consider a cluster with **200 total healthy volumes** across all datanodes and an EC policy of **RS-6-3** (which requires 9 nodes).
+*   `ozone.scm.ec.pipeline.minimum` = **5** (default)
+*   `ozone.scm.ec.pipeline.per.volume.factor` = **1.0** (default)
+
+1.  The volume-based target is: `(1.0 * 200) / 9 = 22`
+2.  The final target is: `max(22, 5) = 22`
+
+SCM will attempt to create and maintain approximately **22** open, RS-6-3 EC pipelines.
+
+**Production Recommendation:**
+
+The default values are a good starting point for most clusters. If you have a very high number of volumes and a write-heavy EC workload, you might consider slightly increasing the `pipeline.per.volume.factor`. Conversely, for read-heavy workloads, the default minimum of 5 pipelines is often sufficient.
+
 ### Enable Intel ISA-L
 
 Intel Intelligent Storage Acceleration Library (ISA-L) is an open-source collection of optimized low-level functions used for

hadoop-hdds/docs/content/start/ProductionDeployment.md

@@ -85,5 +85,5 @@ A typical production Ozone cluster includes the following services:
 ### Ozone Configuration
 
 *   **Monitoring**: Install Prometheus and Grafana for monitoring the Ozone cluster. For audit logs, consider using a log ingestion framework such as the ELK Stack (Elasticsearch, Logstash, and Kibana) with FileBeat, or other similar frameworks. Alternatively, you can use Apache Ranger to manage audit logs.
-*   **Pipeline Limits**: Increase the number of allowed write pipelines to better suit your workload by adjusting `ozone.scm.datanode.pipeline.limit` and `ozone.scm.ec.pipeline.minimum`.
+*   **Pipeline Limits**: Increase the number of allowed write pipelines to better suit your workload by adjusting `ozone.scm.datanode.pipeline.limit` (for Ratis) and `ozone.scm.ec.pipeline.minimum` (for EC).
 *   **Heap Sizes**: Configure sufficient heap sizes for Ozone Manager (OM), Storage Container Manager (SCM), Recon, DataNode, S3 Gateway (S3G), and HttpFs services to ensure stability.