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+~~ Licensed under the Apache License, Version 2.0 (the "License");
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+~~ you may not use this file except in compliance with the License.
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+~~ You may obtain a copy of the License at
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+~~
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+~~ http://www.apache.org/licenses/LICENSE-2.0
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+~~
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+~~ Unless required by applicable law or agreed to in writing, software
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+~~ distributed under the License is distributed on an "AS IS" BASIS,
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+~~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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+~~ See the License for the specific language governing permissions and
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+~~ limitations under the License. See accompanying LICENSE file.
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+
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+ ---
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+ Hadoop Distributed File System-${project.version} - Centralized Cache Management in HDFS
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+ ---
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+ ---
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+ ${maven.build.timestamp}
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+
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+Centralized Cache Management in HDFS
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+
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+ \[ {{{./index.html}Go Back}} \]
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+
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+%{toc|section=1|fromDepth=2|toDepth=4}
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+
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+* {Background}
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+
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+ Normally, HDFS relies on the operating system to cache data it reads from disk.
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+ However, HDFS can also be configured to use centralized cache management. Under
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+ centralized cache management, the HDFS NameNode itself decides which blocks
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+ should be cached, and where they should be cached.
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+
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+ Centralized cache management has several advantages. First of all, it
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+ prevents frequently used block files from being evicted from memory. This is
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+ particularly important when the size of the working set exceeds the size of
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+ main memory, which is true for many big data applications. Secondly, when
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+ HDFS decides what should be cached, it can let clients know about this
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+ information through the getFileBlockLocations API. Finally, when the DataNode
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+ knows a block is locked into memory, it can provide access to that block via
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+ mmap.
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+
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+* {Use Cases}
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+
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+ Centralized cache management is most useful for files which are accessed very
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+ often. For example, a "fact table" in Hive which is often used in joins is a
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+ good candidate for caching. On the other hand, when running a classic
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+ "word count" MapReduce job which counts the number of words in each
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+ document, there may not be any good candidates for caching, since all the
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+ files may be accessed exactly once.
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+
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+* {Architecture}
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+
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+[images/caching.png] Caching Architecture
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+
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+ With centralized cache management, the NameNode coordinates all caching
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+ across the cluster. It receives cache information from each DataNode via the
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+ cache report, a periodic message that describes all the blocks IDs cached on
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+ a given DataNode. The NameNode will reply to DataNode heartbeat messages
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+ with commands telling it which blocks to cache and which to uncache.
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+
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+ The NameNode stores a set of path cache directives, which tell it which files
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+ to cache. The NameNode also stores a set of cache pools, which are groups of
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+ cache directives. These directives and pools are persisted to the edit log
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+ and fsimage, and will be loaded if the cluster is restarted.
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+
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+ Periodically, the NameNode rescans the namespace, to see which blocks need to
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+ be cached based on the current set of path cache directives. Rescans are also
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+ triggered by relevant user actions, such as adding or removing a cache
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+ directive or removing a cache pool.
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+
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+ Cache directives also may specific a numeric cache replication, which is the
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+ number of replicas to cache. This number may be equal to or smaller than the
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+ file's block replication. If multiple cache directives cover the same file
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+ with different cache replication settings, then the highest cache replication
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+ setting is applied.
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+
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+ We do not currently cache blocks which are under construction, corrupt, or
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+ otherwise incomplete. If a cache directive covers a symlink, the symlink
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+ target is not cached.
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+
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+ Caching is currently done on a per-file basis, although we would like to add
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+ block-level granularity in the future.
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+
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+* {Interface}
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+
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+ The NameNode stores a list of "cache directives." These directives contain a
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+ path as well as the number of times blocks in that path should be replicated.
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+
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+ Paths can be either directories or files. If the path specifies a file, that
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+ file is cached. If the path specifies a directory, all the files in the
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+ directory will be cached. However, this process is not recursive-- only the
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+ direct children of the directory will be cached.
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+
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+** {hdfs cacheadmin Shell}
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+
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+ Path cache directives can be created by the <<<hdfs cacheadmin
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+ -addDirective>>> command and removed via the <<<hdfs cacheadmin
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+ -removeDirective>>> command. To list the current path cache directives, use
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+ <<<hdfs cacheadmin -listDirectives>>>. Each path cache directive has a
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+ unique 64-bit ID number which will not be reused if it is deleted. To remove
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+ all path cache directives with a specified path, use <<<hdfs cacheadmin
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+ -removeDirectives>>>.
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+
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+ Directives are grouped into "cache pools." Each cache pool gets a share of
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+ the cluster's resources. Additionally, cache pools are used for
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+ authentication. Cache pools have a mode, user, and group, similar to regular
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+ files. The same authentication rules are applied as for normal files. So, for
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+ example, if the mode is 0777, any user can add or remove directives from the
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+ cache pool. If the mode is 0644, only the owner can write to the cache pool,
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+ but anyone can read from it. And so forth.
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+
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+ Cache pools are identified by name. They can be created by the <<<hdfs
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+ cacheAdmin -addPool>>> command, modified by the <<<hdfs cacheadmin
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+ -modifyPool>>> command, and removed via the <<<hdfs cacheadmin
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+ -removePool>>> command. To list the current cache pools, use <<<hdfs
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+ cacheAdmin -listPools>>>
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+
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+*** {addDirective}
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+
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+ Usage: <<<hdfs cacheadmin -addDirective -path <path> -replication <replication> -pool <pool-name> >>>
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+
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+ Add a new PathBasedCache directive.
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+
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+*--+--+
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+\<path\> | A path to cache. The path can be a directory or a file.
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+*--+--+
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+\<replication\> | The cache replication factor to use. Defaults to 1.
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+*--+--+
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+\<pool-name\> | The pool to which the directive will be added. You must have write permission on the cache pool in order to add new directives.
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+*--+--+
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+
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+*** {removeDirective}
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+
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+ Usage: <<<hdfs cacheadmin -removeDirective <id> >>>
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+
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+ Remove a cache directive.
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+
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+*--+--+
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+\<id\> | The id of the cache directive to remove. You must have write permission on the pool of the directive in order to remove it. To see a list of PathBasedCache directive IDs, use the -listDirectives command.
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+*--+--+
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+
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+*** {removeDirectives}
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+
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+ Usage: <<<hdfs cacheadmin -removeDirectives <path> >>>
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+
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+ Remove every cache directive with the specified path.
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+
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+*--+--+
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+\<path\> | The path of the cache directives to remove. You must have write permission on the pool of the directive in order to remove it. To see a list of cache directives, use the -listDirectives command.
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+*--+--+
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+
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+*** {listDirectives}
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+
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+ Usage: <<<hdfs cacheadmin -listDirectives [-path <path>] [-pool <pool>] >>>
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+
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+ List PathBasedCache directives.
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+
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+*--+--+
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+\<path\> | List only PathBasedCache directives with this path. Note that if there is a PathBasedCache directive for <path> in a cache pool that we don't have read access for, it will not be listed.
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+*--+--+
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+\<pool\> | List only path cache directives in that pool.
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+*--+--+
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+
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+*** {addPool}
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+
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+ Usage: <<<hdfs cacheadmin -addPool <name> [-owner <owner>] [-group <group>] [-mode <mode>] [-weight <weight>] >>>
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+
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+ Add a new cache pool.
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+
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+*--+--+
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+\<name\> | Name of the new pool.
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+*--+--+
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+\<owner\> | Username of the owner of the pool. Defaults to the current user.
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+*--+--+
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+\<group\> | Group of the pool. Defaults to the primary group name of the current user.
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+*--+--+
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+\<mode\> | UNIX-style permissions for the pool. Permissions are specified in octal, e.g. 0755. By default, this is set to 0755.
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+*--+--+
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+\<weight\> | Weight of the pool. This is a relative measure of the importance of the pool used during cache resource management. By default, it is set to 100.
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+*--+--+
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+
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+*** {modifyPool}
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+
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+ Usage: <<<hdfs cacheadmin -modifyPool <name> [-owner <owner>] [-group <group>] [-mode <mode>] [-weight <weight>] >>>
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+
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+ Modifies the metadata of an existing cache pool.
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+
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+*--+--+
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+\<name\> | Name of the pool to modify.
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+*--+--+
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+\<owner\> | Username of the owner of the pool.
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+*--+--+
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+\<group\> | Groupname of the group of the pool.
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+*--+--+
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+\<mode\> | Unix-style permissions of the pool in octal.
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+*--+--+
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+\<weight\> | Weight of the pool.
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+*--+--+
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+
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+*** {removePool}
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+
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+ Usage: <<<hdfs cacheadmin -removePool <name> >>>
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+
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+ Remove a cache pool. This also uncaches paths associated with the pool.
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+
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+*--+--+
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+\<name\> | Name of the cache pool to remove.
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+*--+--+
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+
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+*** {listPools}
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+
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+ Usage: <<<hdfs cacheadmin -listPools [name] >>>
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+
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+ Display information about one or more cache pools, e.g. name, owner, group,
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+ permissions, etc.
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+
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+*--+--+
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+\<name\> | If specified, list only the named cache pool.
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+*--+--+
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+
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+*** {help}
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+
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+ Usage: <<<hdfs cacheadmin -help <command-name> >>>
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+
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+ Get detailed help about a command.
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+
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+*--+--+
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+\<command-name\> | The command for which to get detailed help. If no command is specified, print detailed help for all commands.
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+*--+--+
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+
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+* {Configuration}
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+
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+** {Native Libraries}
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+
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+ In order to lock block files into memory, the DataNode relies on native JNI
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+ code found in <<<libhadoop.so>>>. Be sure to
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+ {{{../hadoop-common/NativeLibraries.html}enable JNI}} if you are using HDFS
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+ centralized cache management.
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+
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+** {Configuration Properties}
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+
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+*** Required
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+
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+ Be sure to configure the following:
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+
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+ * dfs.namenode.caching.enabled
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+
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+ This must be set to true to enable caching. If this is false, the NameNode
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+ will ignore cache reports, and will not ask DataNodes to cache
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+ blocks.
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+
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+ * dfs.datanode.max.locked.memory
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+
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+ The DataNode will treat this as the maximum amount of memory it can use for
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+ its cache. When setting this value, please remember that you will need space
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+ in memory for other things, such as the Java virtual machine (JVM) itself
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+ and the operating system's page cache.
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+
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+*** Optional
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+
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+ The following properties are not required, but may be specified for tuning:
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+
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+ * dfs.namenode.path.based.cache.refresh.interval.ms
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+
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+ The NameNode will use this as the amount of milliseconds between subsequent
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+ path cache rescans. This calculates the blocks to cache and each DataNode
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+ containing a replica of the block that should cache it.
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+
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+ By default, this parameter is set to 300000, which is five minutes.
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+
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+ * dfs.datanode.fsdatasetcache.max.threads.per.volume
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+
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+ The DataNode will use this as the maximum number of threads per volume to
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+ use for caching new data.
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+
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+ By default, this parameter is set to 4.
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+
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+ * dfs.cachereport.intervalMsec
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+
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+ The DataNode will use this as the amount of milliseconds between sending a
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+ full report of its cache state to the NameNode.
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+
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+ By default, this parameter is set to 10000, which is 10 seconds.
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+
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+** {OS Limits}
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+
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+ If you get the error "Cannot start datanode because the configured max
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+ locked memory size... is more than the datanode's available RLIMIT_MEMLOCK
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+ ulimit," that means that the operating system is imposing a lower limit
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+ on the amount of memory that you can lock than what you have configured. To
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+ fix this, you must adjust the ulimit -l value that the DataNode runs with.
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+ Usually, this value is configured in <<</etc/security/limits.conf>>>.
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+ However, it will vary depending on what operating system and distribution
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+ you are using.
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+
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+ You will know that you have correctly configured this value when you can run
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+ <<<ulimit -l>>> from the shell and get back either a higher value than what
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+ you have configured with <<<dfs.datanode.max.locked.memory>>>, or the string
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+ "unlimited," indicating that there is no limit. Note that it's typical for
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+ <<<ulimit -l>>> to output the memory lock limit in KB, but
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+ dfs.datanode.max.locked.memory must be specified in bytes.
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Chris Nauroth
