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https://issues.apache.org/jira/browse/HDFS-14854?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17033490#comment-17033490
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Stephen O'Donnell commented on HDFS-14854:
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Hi [~ayushtkn] - unfortunately we have not been able to run this on anything
outside of a test cluster as yet, but I am very keen to hear reports from
anyone willing to give it a try.
Its really hard to know how much this patch would improve decommission speed
by. On one hand, the speed is still limited by the pending replication queue,
which the decommission monitor places blocks onto. However, the new monitor
places the block on it in a more random fashion to avoid replicating blocks off
the same disk all the time. I also hope the new monitor has less of an overhead
on the namenode as its running, as it does not hold the write lock as often, or
spend time scanning the same blocks over and over.
> 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
> Fix For: 3.3.0
>
> Attachments: 012_to_013_changes.diff,
> 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,
> HDFS-14854.009.patch, HDFS-14854.010.patch, HDFS-14854.011.patch,
> HDFS-14854.012.patch, HDFS-14854.013.patch, HDFS-14854.014.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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