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From "Andrew Wang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5366) recaching improvements
Date Wed, 16 Oct 2013 21:58:42 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5366?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13797319#comment-13797319
] 

Andrew Wang commented on HDFS-5366:
-----------------------------------

One interesting idea from the block replication code is having priorities for replication
work based on the current and expected replication factor. Maybe a "0 of 3" case should be
rescheduled elsewhere more quickly than the 10.5 minute dead datanode interval, while we let
a mild case of "2 of 3" sit.

I don't think this will require tracking our own list of "stale" or "dead" nodes, just a list
of nodes we've already tried for an outstanding request. We reset if we've tried all targets.
I seem to remember the block recovery code or something doing this. Avoiding stale nodes might
also be good enough, if we think that heartbeats are a good proxy for the DN's ability to
cache/uncache. This probably isn't true for uncaching though, since as you've noted, a hung
client could just hold onto a ZCR lease.

> 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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