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https://issues.apache.org/jira/browse/HDFS-14854?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16952009#comment-16952009
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David Mollitor commented on HDFS-14854:
---------------------------------------
Nit: this is not very java-y...
{code:java}
final List<DatanodeDescriptor> toRemove = new ArrayList<>();
...
processMaintenanceNodes(toRemove);
...
// Check if any nodes have reached zero blocks and also update the stats
// exposed via JMX for all nodes still being processed.
checkForCompletedNodes(toRemove);
// Finally move the nodes to their final state if they are ready.
processCompletedNodes(toRemove);
{code}
Better to remove coupling:
{code:java}
final List<DatanodeDescriptor> maintenanceExpiredNodes =
getMaintenanceNodes();
...
final List<DatanodeDescriptor> completedNodes = getCompletedNodes();
Iterable<String> nodesToRemove = Iterables.unmodifiableIterable(
Iterables.concat(maintenanceExpiredNodes , completedNodes));
// Finally move the nodes to their final state if they are ready.
processCompletedNodes(Lists.newArrayList(nodesToRemove));
{code}
> Create improved decommission monitor implementation
> ---------------------------------------------------
>
> Key: HDFS-14854
> URL: https://issues.apache.org/jira/browse/HDFS-14854
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode
> Affects Versions: 3.3.0
> Reporter: Stephen O'Donnell
> Assignee: Stephen O'Donnell
> Priority: Major
> Attachments: Decommission_Monitor_V2_001.pdf, HDFS-14854.001.patch,
> HDFS-14854.002.patch, HDFS-14854.003.patch, HDFS-14854.004.patch,
> HDFS-14854.005.patch, HDFS-14854.006.patch, HDFS-14854.007.patch,
> HDFS-14854.008.patch
>
>
> In HDFS-13157, we discovered a series of problems with the current
> decommission monitor implementation, such as:
> * Blocks are replicated sequentially disk by disk and node by node, and
> hence the load is not spread well across the cluster
> * Adding a node for decommission can cause the namenode write lock to be
> held for a long time.
> * Decommissioning nodes floods the replication queue and under replicated
> blocks from a future node or disk failure may way for a long time before they
> are replicated.
> * Blocks pending replication are checked many times under a write lock
> before they are sufficiently replicate, wasting resources
> In this Jira I propose to create a new implementation of the decommission
> monitor that resolves these issues. As it will be difficult to prove one
> implementation is better than another, the new implementation can be enabled
> or disabled giving the option of the existing implementation or the new one.
> I will attach a pdf with some more details on the design and then a version 1
> patch shortly.
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