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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5399) Revisit SafeModeException and corresponding retry policies
Date Tue, 04 Feb 2014 19:32:17 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5399?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13891066#comment-13891066
] 

Todd Lipcon commented on HDFS-5399:
-----------------------------------

Catching up on this issue now, so apologies if I've missed some context in my reading of the
discussion.

- One of the issues is that the SBN may be in Safe Mode while it's tailing. When it becomes
active, it has the latest edits and can come out of safemode, but still goes through the extension.
The original reason for the extension to prevent a replication storm in the case that the
NN has only one replica of all the blocks, but several DNs haven't yet reported. In the SBN
case, since we've already been running in standby mode for a while, it seems unlikely that
the extension is necessary. Maybe we should consider changing the extension so that, if we
don't have a significant number of under-replicated blocks, we don't go through the extension?

- Regardless, we should limit the number of retries as Jing proposed above. Retrying indefinitely
should never be our default. How about we introduce a configuration here and default to ~30sec
of retries? Those who want to retry forever could reconfigure to a longer time period.

> Revisit SafeModeException and corresponding retry policies
> ----------------------------------------------------------
>
>                 Key: HDFS-5399
>                 URL: https://issues.apache.org/jira/browse/HDFS-5399
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>    Affects Versions: 2.3.0
>            Reporter: Jing Zhao
>            Assignee: Jing Zhao
>
> Currently for NN SafeMode, we have the following corresponding retry policies:
> # In non-HA setup, for certain API call ("create"), the client will retry if the NN is
in SafeMode. Specifically, the client side's RPC adopts MultipleLinearRandomRetry policy for
a wrapped SafeModeException when retry is enabled.
> # In HA setup, the client will retry if the NN is Active and in SafeMode. Specifically,
the SafeModeException is wrapped as a RetriableException in the server side. Client side's
RPC uses FailoverOnNetworkExceptionRetry policy which recognizes RetriableException (see HDFS-5291).
> There are several possible issues in the current implementation:
> # The NN SafeMode can be a "Manual" SafeMode (i.e., started by administrator through
CLI), and the clients may not want to retry on this type of SafeMode.
> # Client may want to retry on other API calls in non-HA setup.
> # We should have a single generic strategy to address the mapping between SafeMode and
retry policy for both HA and non-HA setup. A possible straightforward solution is to always
wrap the SafeModeException in the RetriableException to indicate that the clients should retry.



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