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https://issues.apache.org/jira/browse/HDFS-3863?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13444461#comment-13444461
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Todd Lipcon commented on HDFS-3863:
-----------------------------------
I decided to punt this aspect to a follow-up patch:
{quote}
This alone is enough for a good sanity check. If we want to also support
reading the committed transactions while in-progress, it's not quite sufficient
– the last batch of transactions will never be readable if the NN stops writing
new batches for a protracted period of time. To solve this, we can add a timer
thread to the client which periodically (eg once or twice a second) sends an
RPC to update the committed-txid on all of the nodes. The periodic timer will
also have the nice property of causing a NN which has been fenced to abort
itself even if no write transactions are taking place.
{quote}
> QJM: track last "committed" txid
> --------------------------------
>
> Key: HDFS-3863
> URL: https://issues.apache.org/jira/browse/HDFS-3863
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: ha
> Affects Versions: QuorumJournalManager (HDFS-3077)
> Reporter: Todd Lipcon
> Assignee: Todd Lipcon
> Attachments: hdfs-3863-prelim.txt
>
>
> Per some discussion with [~stepinto]
> [here|https://issues.apache.org/jira/browse/HDFS-3077?focusedCommentId=13422579&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13422579],
> we should keep track of the "last committed txid" on each JournalNode. Then
> during any recovery operation, we can sanity-check that we aren't asked to
> truncate a log to an earlier transaction.
> This is also a necessary step if we want to support reading from in-progress
> segments in the future (since we should only allow reads up to the commit
> point)
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