sodonnel opened a new pull request #1605:
URL: https://github.com/apache/ozone/pull/1605


   ## What changes were proposed in this pull request?
   
   When a datanode is started, it must read some metadata from all the 
Containers. Part of that metadata is stored in RocksDB, so the startup process 
involves opening each rocksDB and closing it again.
   
   Testing on a dense node, with 45 high performance spinning disks and 200K 
containers, I saw about 75ms on average to open each RockDB. Further testing 
demonstrated that if we open RockDB read only, the average open time is about 
35ms.
   
   At startup time, the DBs are only read, and never written, so opening read 
only is fine. HDDS-4427 already ensures these opened DBs are not cached.
   
   This change adds an "openReadOnly" flag to RDBStore and the related classes 
to pass it down the stack, allowing the DB to be opened read only.
   
   ## What is the link to the Apache JIRA
   
   https://issues.apache.org/jira/browse/HDDS-4488
   
   ## How was this patch tested?
   
   Currently existing unit tests.
   


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