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https://issues.apache.org/jira/browse/HDFS-5366?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13797468#comment-13797468
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Colin Patrick McCabe commented on HDFS-5366:
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The thing about block replication is, if you lose all copies of the block, you
have a problem. For us, if we lose all cache replicas, it's not a big deal.
It's not obvious that a block which has 2 cached replicas out of 3 requested
should be given lower priority than one with 0 out of 3. Maybe the 2/3 block
is just that much more important. It will depend on the which pools the
requests came from. I guess we'll have to do that as part of the effort to do
pool quotas.
> recaching improvements
> ----------------------
>
> Key: HDFS-5366
> URL: https://issues.apache.org/jira/browse/HDFS-5366
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: namenode
> Affects Versions: HDFS-4949
> Reporter: Colin Patrick McCabe
> Assignee: Colin Patrick McCabe
>
> There are a few things about our HDFS-4949 recaching strategy that could be
> improved.
> * We should monitor the DN's maximum and current mlock'ed memory consumption
> levels, so that we don't ask the DN to do stuff it can't.
> * We should not try to initiate caching on stale DataNodes (although we
> should not recache things stored on such nodes until they're declared dead).
> * We might want to resend the {{DNA_CACHE}} or {{DNA_UNCACHE}} command a few
> times before giving up. Currently, we only send it once.
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