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From "Bikas Saha (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3212) Persist the epoch received by the JournalService
Date Thu, 05 Apr 2012 23:42:23 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13247854#comment-13247854
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Bikas Saha commented on HDFS-3212:
----------------------------------

I have been trying to read ZAB and re-read PAXOS before I make some comments on some of the
epoch stuff.
At first glance, it seems to me that some of these operations need to be atomic. I havent
caught up with HDFS-3077 but I remember Tod clarifying to an example of mine by saying that
edit log segments are relevant in the context of an epoch. So 2 edit logs with same txid but
can be differentiated using epochs. In that case, it makes sense tying the epoch to segment
relation in the roll via 1 above. Because then creating a segment and attaching it to an epoch
would be 1 operation to the extent rolling is 1 operation.
2. might be less optimal because now it consists of 2 operations. 1) rolling the log and creating
a new segment 2) updating a metadata file.
However, my understanding of rolling might be incomplete. So please take this with the necessary
pinch of salt :P
                
> Persist the epoch received by the JournalService
> ------------------------------------------------
>
>                 Key: HDFS-3212
>                 URL: https://issues.apache.org/jira/browse/HDFS-3212
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: ha, name-node
>    Affects Versions: Shared journals (HDFS-3092)
>            Reporter: Suresh Srinivas
>
> epoch received over JournalProtocol should be persisted by JournalService.

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