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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-10967) Add configuration for BlockPlacementPolicy to avoid near-full DataNodes
Date Mon, 10 Oct 2016 23:27:20 GMT

    [ https://issues.apache.org/jira/browse/HDFS-10967?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15563882#comment-15563882

Konstantin Shvachko commented on HDFS-10967:

This could be good optimization of placement policy for other than the first replicas when
some nodes are close to full.
This should only be turned on though in heterogeneous environment. With homogeneous nodes
where node usage is balanced this will be just a performance overhead.
Few suggestions on the patch
# Set the default for remaining capacity threshold that triggers new behavior to {{0.00}}.
Homogeneous clusters should not require reconfiguration.
# The config variable is more like a "threshold" rather than a "factor" as in {{considerLoad}}.
So may be call it {{dfs.namenode.replication.considerCapacity.threshold}}.
# I would suggest to use only one configuration variable. The Boolean {{considerCapacity}}
is essentially redundant.
# Would be good to have a JavaDoc for the new config variable and for the method, where the
feature is triggered.
# Did you try to add {{isNearFull()}} call into {{isGoodDatanode()}}? Then you will not need
to retry 3 times.
# JavaDoc for the unit test would be also useful.

A separate discussion point whether we should add a separate admin command for every configuration
nob we introduce? This doesn't feel right. May be we should have a command, say {{refreshConfiguration()}},
which re-reads config, updates variables and logs what it updated. The main problem with admin
command is that it does not change configuration and when you restart the service or fail
it over you loose the set value. So may be anything that is controlled by configuration should
be updated through configuration and the {{refreshConfiguration()}} call. We could have merged
this with {{setBalancerBandwidth()}} some time.
Alternatively, we can use failover to take effect of new configuration values. Then we don't
need new admin commands at all.

> Add configuration for BlockPlacementPolicy to avoid near-full DataNodes
> -----------------------------------------------------------------------
>                 Key: HDFS-10967
>                 URL: https://issues.apache.org/jira/browse/HDFS-10967
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: namenode
>            Reporter: Zhe Zhang
>            Assignee: Zhe Zhang
>              Labels: balancer
>         Attachments: HDFS-10967.00.patch, HDFS-10967.01.patch, HDFS-10967.02.patch, HDFS-10967.03.patch
> Large production clusters are likely to have heterogeneous nodes in terms of storage
capacity, memory, and CPU cores. It is not always possible to proportionally ingest data into
DataNodes based on their remaining storage capacity. Therefore it's possible for a subset
of DataNodes to be much closer to full capacity than the rest.
> This heterogeneity is most likely rack-by-rack -- i.e. _m_ whole racks of low-storage
nodes and _n_ whole racks of high-storage nodes. So It'd be very useful if we can lower the
chance for those near-full DataNodes to become destinations for the 2nd and 3rd replicas.

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