runzhiwang opened a new pull request #709: HDDS-3244. Improve write efficiency by opening RocksDB only once URL: https://github.com/apache/hadoop-ozone/pull/709 ## What changes were proposed in this pull request? What's the problem ? 1. when datanode create each container, a new RocksDB instance will be [created|https://github.com/apache/hadoop-ozone/blob/master/hadoop-hdds/container-service/src/main/java/org/apache/hadoop/ozone/container/keyvalue/helpers/KeyValueContainerUtil.java#L76], but then the created RocksDB was [closed|https://github.com/apache/hadoop-ozone/blob/master/hadoop-hdds/container-service/src/main/java/org/apache/hadoop/ozone/container/keyvalue/helpers/KeyValueContainerUtil.java#L83], until PutBlock the RocsDB will be [opend|https://github.com/apache/hadoop-ozone/blob/master/hadoop-hdds/container-service/src/main/java/org/apache/hadoop/ozone/container/common/utils/ContainerCache.java#L123] again, so the RocksDB was open twice. [RocksDB.open|https://github.com/apache/hadoop-ozone/blob/master/hadoop-hdds/framework/src/main/java/org/apache/hadoop/hdds/utils/RocksDBStore.java#L68] cost about 150ms-200ms, it's a waste to open it twice. 2. Besides, as the image shows, when leader datanode open RocksDB in WriteData , the follower can not open RocksDB until the leader finish it. So the whole `ObjectPoint.Put` stage cost 3 * cost(RocksDB.open) = 600ms to open RocksDB.  3. When upload a 3KB file five times, the average cost is 912ms.   How to fix it ? 1. Open RocksDB in CommitData rather than WriteData, because leader datanode and 2 follower datanodes can open RocksDB in parallel in CommitData. 2. Put the RocksDB handler into cache after open it in CommitData, to avoid open it again when PutBlock. 3. So the whole `ObjectPoint.Put` stage cost 1 * cost(RocksDB.open) = 200ms to open RocksDB.  4. When upload a 3KB file five times, the average cost is 516ms, improve about 44%.  ## What is the link to the Apache JIRA https://issues.apache.org/jira/projects/HDDS/issues/HDDS-3244 ## How was this patch tested? I will change the UTs to pass the CI.
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