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https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13399741#comment-13399741
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Brandon Li commented on HDFS-3077:
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Hi Todd,
I just read the new section but not the implementation yet. Please correct me
if I am wrong. Looks like it could be a possible improvement: if the
JournalNode who already received a prepare RPC with higher newEpoch number
(possible?) can inform the writer(proposer), the writer can exit earlier in
step 2 "Choosing a recovery".
In step 3 "Accept RPC", I assume the URL that the writer sends to all the JNs
is the URL of one JN which responded in step2. If that JN becomes inaccessible
immediately, and thus other JNs can't sync themselves by downloading the
finalized segment from that JN, the recovery process could be stuck?
If it could be stuck in step 3, an alternate way to sync lagging JNs is to let
them contact other quorum number JNs to download the finalized segments. Given
the requirement "All loggers must finalize the segment to the same length and
contents", all the finalized segment with the same name should be identical in
all JNs. Therefore, the lagging JN can download it from any other JN as long as
that JN has the file.
> Quorum-based protocol for reading and writing edit logs
> -------------------------------------------------------
>
> Key: HDFS-3077
> URL: https://issues.apache.org/jira/browse/HDFS-3077
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: ha, name-node
> Reporter: Todd Lipcon
> Assignee: Todd Lipcon
> Attachments: hdfs-3077-partial.txt, qjournal-design.pdf,
> qjournal-design.pdf
>
>
> Currently, one of the weak points of the HA design is that it relies on
> shared storage such as an NFS filer for the shared edit log. One alternative
> that has been proposed is to depend on BookKeeper, a ZooKeeper subproject
> which provides a highly available replicated edit log on commodity hardware.
> This JIRA is to implement another alternative, based on a quorum commit
> protocol, integrated more tightly in HDFS and with the requirements driven
> only by HDFS's needs rather than more generic use cases. More details to
> follow.
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