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From "Erik Krogen (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-13873) ObserverNode should reject read requests when it is too far behind.
Date Tue, 11 Dec 2018 17:18:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-13873?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16717580#comment-16717580

Erik Krogen commented on HDFS-13873:

I like your logic, [~shv], but I think it could still result in the live-lock issue [~daryn]
was originally concerned with if you have a very read-heavy cluster with very few writes,
if you have either a malicious client or a lag in edit log tailing. Let's assume there is
only 100 write/s and {{client.LastSeenId - observer.LastWrittenId == 10000}} (because the
client is malicious or because the observer fell far behind), the server will not reject the
request, but the observer will take a long time before it serves the request, and if there
are multiple such request it could fill all of the handlers. I think ideally we also need
to add some sort of threshold based on time the request has waited or number of times the
request has been pushed back into the queue.

> ObserverNode should reject read requests when it is too far behind.
> -------------------------------------------------------------------
>                 Key: HDFS-13873
>                 URL: https://issues.apache.org/jira/browse/HDFS-13873
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: hdfs-client, namenode
>    Affects Versions: HDFS-12943
>            Reporter: Konstantin Shvachko
>            Assignee: Chao Sun
>            Priority: Major
> Add a server-side threshold for ObserverNode to reject read requests when it is too far

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