[
https://issues.apache.org/jira/browse/HDFS-7979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14378510#comment-14378510
]
Andrew Wang commented on HDFS-7979:
-----------------------------------
Sorry if I wasn't clear about this, but "safety" here is about not repeating.
The monotonic time is the time since DN startup, so if the machine goes down
and comes back up, it'll be in the same range of monotonic times. Agree that it
doesn't reset if just the DN process is restarted.
bq. When using the monotonic time, two block reports cannot get the same ID
since the monotonic time is always increasing. We don't have the same guarantee
here. Admittedly, the chances of a repeat are extremely low. But previously
they were effectively 0, and now they're nonzero.
This seems less likely to repeat compared to monotonic time, because of machine
reboots.
bq. If the datanode is taken down and restarted, the monotonic time will still
be higher than before. And so the current behavior makes it easy to see from
the logs that block report N+1 came after block report N, even if there was a
datanode restart in between. We don't have this behavior with a random number
generated on datanode start.
This is true, but not sure how useful this is. If there's logging on both the
DN and NN, that would also be a way of determining ordering.
> Initialize block report IDs with a random number
> ------------------------------------------------
>
> Key: HDFS-7979
> URL: https://issues.apache.org/jira/browse/HDFS-7979
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 2.7.0
> Reporter: Andrew Wang
> Assignee: Andrew Wang
> Priority: Minor
> Attachments: HDFS-7979.001.patch
>
>
> Right now block report IDs use system nanotime. This isn't that random, so
> let's start it at a random number for some more safety.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
