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From "Jing Zhao (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6184) Better health check from ZKFC
Date Wed, 02 Apr 2014 17:55:18 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6184?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13957950#comment-13957950

Jing Zhao commented on HDFS-6184:

After the change in HDFS-5064 the checkpointer in SBN will hold a long read lock? But since
DNs' rpc calls can lead to write operations in SBN the issue in #1 is still there.

> Better health check from ZKFC
> -----------------------------
>                 Key: HDFS-6184
>                 URL: https://issues.apache.org/jira/browse/HDFS-6184
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: namenode
>            Reporter: Ming Ma
> We have seen several false positives in terms of when ZKFC considers NN to be unhealthy.
Some of these triggers unnecessary failover. Examples,
> 1. SBN checkpoint caused ZKFC's RPC call into NN timeout. The consequence isn't bad;
just that SBN will quit ZK membership and rejoin it later. But it is unnecessary. The reason
is checkpoint acquires NN global write lock and all rpc requests are blocked. Even though
HAServiceProtocol.monitorHealth doesn't need to acquire NN lock; it still needs to user service
rpc queue.
> 2. When ANN is busy, sometimes the global lock can block other requests. ZKFC's RPC call
timeout. This will trigger failover. The question is even if after the failover, the new ANN
might run into similar issue.
> We can increase ZKFC to NN timeout value to mitigate this to some degree. If ZKFC can
be more accurate in judgment if NN is health or not and can predict the failover will help,
that will be useful. For example, we can,
> 1. Have ZKFC made decision based on NN thread dump.
> 2. Have a dedicated rpc pool for ZKFC > NN. Given health check doesn't need to acquire
NN global lock; so it can go through even if NN is doing checkpointing or very busy.
> Any comments?

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