You are viewing a plain text version of this content. The canonical link for it is here.
Posted to common-issues@hadoop.apache.org by "Daryn Sharp (Commented) (JIRA)" <ji...@apache.org> on 2012/02/09 15:39:59 UTC
[jira] [Commented] (HADOOP-8047) CachedDNSToSwitchMapping caches
negative results forever
[ https://issues.apache.org/jira/browse/HADOOP-8047?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13204554#comment-13204554 ]
Daryn Sharp commented on HADOOP-8047:
-------------------------------------
This is actually a general problem if the cache is in a persistent daemon like the namenode. Transient DNS problems can cause false entries in the negative cache.
> CachedDNSToSwitchMapping caches negative results forever
> --------------------------------------------------------
>
> Key: HADOOP-8047
> URL: https://issues.apache.org/jira/browse/HADOOP-8047
> Project: Hadoop Common
> Issue Type: Bug
> Components: util
> Affects Versions: 0.23.0, 0.24.0, 1.0.0
> Reporter: Steve Loughran
> Priority: Trivial
>
> This is very minor, just worth filing in JIRA unless someone wants to rethink topology caching for a dynamic world.
> # The CachedDNSToSwitchMapping caches the results from all relayed DNS queries.
> # The DNS script mapper returns the default rack for all unknown entries (or when the script fails)
> # The Cache stores this in its map and never re-resolves it.
> As a result, if a node is added to a live cluster that the existing script cannot resolve, then it won't get assigned to a rack unless the script is updated before the rack map is resolved.
> This isn't usually that important, it just means "update your scripts before adding new racks". Perhaps there should be a page on that activity, "runbook and checklist for adding new servers and racks".
> Where it would matter if anyone started playing with dynamic topologies, but in that situation the cached mapping itself would become the liability, as it assumes that servers never switch switches in a live system: the topology is static for existing nodes.
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators: https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira