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  1. Hadoop HDFS
  2. HDFS-16262

Async refresh of cached locations in DFSInputStream



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      HDFS-15119 added the ability to invalidate cached block locations in DFSInputStream. As written, the feature will affect all DFSInputStreams regardless of whether they need it or not. The invalidation also only applies on the next request, so the next request will pay the cost of calling openInfo before reading the data.

      I'm working on a feature for HBase which enables efficient healing of locality through Balancer-style low level block moves (HBASE-26250). I'd like to utilize the idea started in HDFS-15119 in order to update DFSInputStreams after blocks have been moved to local hosts.

      I was considering using the feature as is, but some of our clusters are quite large and I'm concerned about the impact on the namenode:

      • We have some clusters with over 350k StoreFiles, so that'd be 350k DFSInputStreams. With such a large number and very active usage, having the refresh be in-line makes it too hard to ensure we don't DDOS the NameNode.
      • Currently we need to pay the price of openInfo the next time a DFSInputStream is invoked. Moving that async would minimize the latency hit. Also, some StoreFiles might be far less frequently accessed, so they may live on for a long time before ever refreshing. We'd like to be able to know that all DFSInputStreams are refreshed by a given time.
      • We may have 350k files, but only a small percentage of them are ever non-local at a given time. Refreshing only if necessary will save a lot of work.

      In order to make this as painless to end users as possible, I'd like to:

      • Update the implementation to utilize an async thread for managing refreshes. This will give more control over rate limiting across all DFSInputStreams in a DFSClient, and also ensure that all DFSInputStreams are refreshed.
      • Only refresh files which are lacking a local replica or have known deadNodes to be cleaned up




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              bbeaudreault Bryan Beaudreault
              bbeaudreault Bryan Beaudreault
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