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. This happens when datanode create container. I split the `HddsDispatcher.WriteChunk` into `HddsDispatcher.WriteData` and `HddsDispatcher.CommitData` as the code shows, to show the cost of them in jaeger UI.  2. 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) in `HddsDispatcher.WriteData` , 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 `HddsDispatcher.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 in each datanode. And the RocksDB was not used until `HddsDispatcher.PutBlock`. 3. Besides, as the image shows, when leader datanode open RocksDB in `HddsDispatcher.WriteData` , 2 follower datanodes can not open RocksDB until the leader finish it. So the whole write cost 3 * cost(RocksDB.open) = 600ms to open RocksDB.  4. When upload a 3KB file five times, the average cost is 912ms.  How to fix it ? 1. Open RocksDB in `HddsDispatcher.CommitData` rather than `HddsDispatcher.WriteData`, because leader datanode and 2 follower datanodes can open RocksDB in parallel in `HddsDispatcher.CommitData`. 2. Put the RocksDB handler into cache after open it in `HddsDispatcher.CommitData`, to avoid open it again when `HddsDispatcher.PutBlock`. 3. So the whole write 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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