ChenSammi commented on code in PR #7529: URL: https://github.com/apache/ozone/pull/7529#discussion_r1875663504
########## hadoop-hdds/docs/content/feature/Short-Circuit-Read.md: ########## @@ -0,0 +1,76 @@ +--- +title: "Short Circuit Local Read in Datanode" +weight: 2 +menu: + main: + parent: Features +summary: Introduction to Ozone Datanode Short Circuit Local Read Feature +--- +<!--- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to You under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. +--> + +Current in Ozone, client reads data over GRPC from Datanode. When the client asks the DataNode to read a file, the DataNode reads that file off of the disk and sends the data to the client over GRPC connection. + +This “short-circuit” local read feature will bypass the DataNode, allowing the client to read the file from local disk directly when the client is co-located with the data on the same server. + +Short-circuit local read can provide a substantial performance boost to many applications, by removing the overhead of network communication. + +## Prerequisite + +Short-circuit local reads make use of a UNIX domain socket. This is a special path in the filesystem that allows the client and the DataNodes to communicate. + +The Hadoop native library "libhadoop.so" provides the support to use the Unix domain socket. Please refer to Native Libraries ("https://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-common/NativeLibraries.html";) for details of this library. + +Before enabling short-circuit local reads, you must have a proper libhadoop.so, and make sure it's under the directory where Java can find and load it through "System.loadLibrary()" call. + +The paths that Java will search for libraries are specified by the "java.library.path" property. The default value of "java.library.path" depends on the OS and Java version. For example, on Linux with OpenJDK 8 it is `/usr/java/packages/lib/amd64:/usr/lib64:/lib64:/lib:/usr/lib`. + +Command "ozone checknative" can be used to detect whether libhadoop.so can be loaded successfully by Ozone service. + +## Configuration + +Short-circuit local reads need to be configured on both the DataNode and the client. By default, it is disabled. + +```XML +<property> + <name>ozone.client.read.short-circuit</name> + <value>false</value> + <description>Disable or enable the short-circuit local read feature.</description> +</property> +``` + +It makes use of a UNIX domain socket, a special path in the filesystem. You will need to set a path to this socket. + +```XML +<property> + <name>ozone.domain.socket.path</name> + <value>/var/lib/ozone_dn_socket</value> + <description>The path used to create domain socket.</description> +</property> +``` + +The DataNode needs to be able to create this path. On the other hand, it should not be possible for any user except the Ozone user(user who launches Ozone service) or root to create this path. For this reason, paths under /var/run or /var/lib are often used， just like the current default value "/var/lib/ozone_dn_socket". + +If you configure the "ozone.domain.socket.path" to other value, for example "/dir1/dir2/ozone_dn_socket", please make sure that both dir1 and dir2 are exiting directories, and there is no ozone_dn_socket under dir2. ozone_dn_socket will be created by Ozone Datanode later during Datanode start. + +### Security Consideration + +To ensure data security and integrity, Ozone will follow the rule as Hadoop ("https://cwiki.apache.org/confluence/display/HADOOP2/SocketPathSecurity";) on "ozone.domain.socket.path" path permission check. Review Comment: The same libhadoop.so is used as Hadoop. No re-implementation of native code part. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
