[GitHub] [hadoop] elek commented on a change in pull request #1196: HDDS-1881. Design doc: decommissioning in Ozone

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elek commented on a change in pull request #1196: HDDS-1881. Design doc: decommissioning in Ozone
URL: https://github.com/apache/hadoop/pull/1196#discussion_r309632593
 
 

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 File path: hadoop-hdds/docs/content/design/decommissioning.md
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+---
+title: Decommissioning in Ozone
+summary: Formal process to shut down machines in a safe way after the required replications.
+date: 2019-07-31
+jira: HDDS-1881
+status: current
+author: Anu Engineer, Marton Elek, Stephen O'Donnell 
+---
+
+
+# Abstract 
+
+The goal of decommissioning is to turn off a selected set of machines without data loss. It may or may not require to move the existing replicas of the containers to other nodes.
+
+There are two main classes of the decommissioning:
+
+ * __Maintenance mode__: where the node is expected to be back after a while. It may not require replication of containers if enough replicas are available from other nodes (as we expect to have the current replicas after the restart.)
+
+ * __Decommissioning__: where the node won't be started again. All the data should be replicated according to the current replication rules.
+
+Goals:
+
+ * Decommissioning can be canceled any time
+ * The progress of the decommissioning should be trackable
+ * The nodes under decommissioning / maintenance mode should not been used for new pipelines / containers
+ * The state of the datanodes should be persisted / replicated by the SCM (in HDFS the decommissioning info exclude/include lists are replicated manually by the admin). If datanode is marked for decommissioning this state be available after SCM and/or Datanode restarts.  
+ * We need to support validations before decommissioing (but the violations can be ignored by the admin).
+ * The administrator should be notified when a node can be turned off.
+ * The maintenance mode can be time constrained: if the node marked for maintenance for 1 week and the node is not up after one week, the containers should be considered as lost (DEAD node) and should be replicated.
+
+# Introduction
+
+Ozone is a highly available file system that relies on commodity hardware. In other words, Ozone is designed to handle failures of these nodes all the time.
+
+The Storage Container Manager(SCM) is designed to monitor the node health and replicate blocks and containers as needed.
+
+At times, Operators of the cluster can help the SCM by giving it hints. When removing a datanode, the operator can provide a hint. That is, a planned failure of the node is coming up, and SCM can make sure it reaches a safe state to handle this planned failure.
+
+Some times, this failure is transient; that is, the operator is taking down this node temporarily. In that case, we can live with lower replica counts by being optimistic.
+
+Both of these operations, __Maintenance__, and __Decommissioning__ are similar from the Replication point of view. In both cases, and the user instructs us on how to handle an upcoming failure.
+
+Today, SCM (*Replication Manager* component inside SCM) understands only one form of failure handling. This paper extends Replica Manager failure modes to allow users to request which failure handling model to be adopted(Optimistic or Pessimistic).
+
+Based on physical realities, there are two responses to any perceived failure, to heal the system by taking corrective actions or ignore the failure since the actions in the future will heal the system automatically.
+
+## User Experiences (Decommissioning vs Maintenance mode)
+
+From the user's point of view, there are two kinds of planned failures that the user would like to communicate to Ozone.
+
+The first kind is when a 'real' failure is going to happen in the future. This 'real' failure is the act of decommissioning. We denote this as "decommission" throughout this paper. The response that the user wants is SCM/Ozone to make replicas to deal with the planned failure.
+
+The second kind is when the failure is 'transient.' The user knows that this failure is temporary and cluster in most cases can safely ignore this issue. However, if the transient failures are going to cause a failure of availability; then the user would like the Ozone to take appropriate actions to address it.  An example of this case, is if the user put 3 data nodes into maintenance mode and switched them off.
+
+The transient failure can violate the availability guarantees of Ozone; Since the user is telling us not to take corrective actions. Many times, the user does not understand the impact on availability while asking Ozone to ignore the failure.
+
+So this paper proposes the following definitions for Decommission and Maintenance of data nodes.
+
+__Decommission__ of a data node is deemed to be complete when SCM/Ozone completes the replica of all containers on decommissioned data node to other data nodes.That is, the expected count matches the healthy count of containers in the cluster.
+
+__Maintenance mode__ of a data node is complete if Ozone can guarantee at least one copy of every container is available in other healthy data nodes.
 
 Review comment:
   Thanks. I added it to the latest version (under "the proposed solution") and added this use case as an example.

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