HDFS-12874. Documentation for provided storage. Contributed by Virajith Jalaparti
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Chris Douglas
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<description>
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The class that is used to specify the input format of the blocks on
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provided storages. The default is
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org.apache.hadoop.hdfs.server.common.TextFileRegionAliasMap which uses
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org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.TextFileRegionAliasMap which uses
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file regions to describe blocks. The file regions are specified as a
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delimited text file. Each file region is a 6-tuple containing the
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block id, remote file path, offset into file, length of block, the
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<!---
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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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http://www.apache.org/licenses/LICENSE-2.0
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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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HDFS Provided Storage
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=====================
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Provided storage allows data *stored outside HDFS* to be mapped to and addressed
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from HDFS. It builds on [heterogeneous storage](./ArchivalStorage.html) by
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introducing a new storage type, `PROVIDED`, to the set of media in a datanode.
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Clients accessing data in
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`PROVIDED` storages can cache replicas in local media, enforce HDFS invariants
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(e.g., security, quotas), and address more data than the cluster could persist
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in the storage attached to DataNodes. This architecture is particularly useful
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in scenarios where HDFS clusters are ephemeral (e.g., cloud scenarios), and/or
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require to read data that lives in other storage systems (e.g., blob stores).
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Provided storage is an experimental feature in HDFS.
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<!-- MACRO{toc|fromDepth=0|toDepth=3} -->
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Introduction
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------------
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As of this writing, support for mounting external storage as `PROVIDED` blocks
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is limited to creating a *read-only image* of a remote namespace that implements the
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`org.apache.hadoop.fs.FileSystem` interface, and starting a NameNode
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to serve the image. Specifically, reads from a snapshot of a remote namespace are
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supported. Adding a remote namespace to an existing/running namenode, refreshing the
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remote snapshot, unmounting, and writes are not available in this release. One
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can use [ViewFs](./ViewFs.html) and [RBF](HDFSRouterFederation.html) to
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integrate namespaces with `PROVIDED` storage into an existing deployment.
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Creating HDFS Clusters with `PROVIDED` Storage
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----------------------------------------------
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One can create snapshots of the remote namespace using the `fs2img` tool. Given
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a path to a remote `FileSystem`, the tool creates an _image_ mirroring the
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namespace and an _alias map_ that maps blockIDs in the generated image to a
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`FileRegion` in the remote filesystem. A `FileRegion` contains sufficient information to
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address a fixed sequence of bytes in the remote `FileSystem` (e.g., file, offset, length)
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and a nonce to verify that the region is unchanged since the image was generated.
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After the NameNode image and alias map are created, the NameNode and DataNodes
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must be configured to consistently reference this address space. When a DataNode
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registers with an attached, `PROVIDED` storage, the NameNode considers all the
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external blocks to be addressable through that DataNode, and may begin to direct
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clients to it. Symmetrically, the DataNode must be able to map every block in
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the `PROVIDED` storage to remote data.
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Deployment details vary depending on the configured alias map implementation.
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### `PROVIDED` Configuration
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Each NameNode supports one alias map. When `PROVIDED` storage is enabled,
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the storage ID configured on the NameNode and DataNodes must match.
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All other details are internal to the alias map implementation.
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The configuration to enable `PROVIDED` storage is as follows.
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The configuration options available for the alias map implementations are
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available below.
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```xml
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<configuration>
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<property>
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<name>dfs.namenode.provided.enabled</name>
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<value>true</value>
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<description>Enabled provided storage on the Namenode</description>
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</property>
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<property>
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<name>dfs.datanode.data.dir</name>
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<value>[DISK]/local/path/to/blocks/, [PROVIDED]remoteFS://remoteFS-authority/path/to/data/</value>
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</property>
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<property>
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<name>dfs.provided.storage.id</name>
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<value>DS-PROVIDED</value>
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<description>The storage ID used for provided storages in the cluster.</description>
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</property>
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<property>
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<name>dfs.provided.aliasmap.class</name>
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<value>org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.TextFileRegionAliasMap</value>
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</property>
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</configuration>
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```
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### fs2img tool
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The `fs2img` tool "walks" over a remote namespace by recursively enumerating
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children of a remote URI to produce an FSImage. Some attributes can be
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controlled by plugins, such as owner/group mappings from the remote filesystem
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to HDFS and the mapping of files to HDFS blocks.
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The various options available in running the tool are:
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| Option | Property | Default | Description |
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|:------------------------|:--------------------------- |:----------------- |:---- |
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| `-o`, `--outdir` | dfs.namenode.name.dir | file://${hadoop.tmp.dir}/dfs/name | Output directory |
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| `-b`, `--blockclass` | dfs.provided.aliasmap.class | NullBlocksMap | Block output class |
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| `-u`, `--ugiclass` | hdfs.image.writer.ugi.class | SingleUGIResolver | UGI resolver class |
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| `-i`, `--blockidclass` | hdfs.image.writer.blockresolver.class | FixedBlockResolver | Block resolver class |
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| `-c`, `--cachedirs` | hdfs.image.writer.cache.entries | 100 | Max active dirents |
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| `-cid`, `--clusterID` | | | Cluster ID |
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| `-bpid`, `--blockPoolID`| | | Block pool ID |
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#### Examples
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Assign all files to be owned by "rmarathe", write to gzip compressed text:
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```
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hadoop org.apache.hadoop.hdfs.server.namenode.FileSystemImage \
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-Dhdfs.image.writer.ugi.single.user=rmarathe \
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-Ddfs.provided.aliasmap.text.codec=gzip \
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-Ddfs.provided.aliasmap.text.write.dir=file:///tmp/
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-b org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.TextFileRegionAliasMap \
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-u org.apache.hadoop.hdfs.server.namenode.SingleUGIResolver \
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-o file:///tmp/name \
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hdfs://afreast/projects/ydau/onan
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```
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Assign ownership based on a custom `UGIResolver`, in LevelDB:
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```
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hadoop org.apache.hadoop.hdfs.server.namenode.FileSystemImage \
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-Ddfs.provided.aliasmap.leveldb.path=file:///path/to/leveldb/map/dingos.db \
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-b org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.LevelDBFileRegionAliasMap \
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-o file:///tmp/name \
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-u CustomResolver \
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hdfs://enfield/projects/ywqmd/incandenza
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```
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Alias Map Implementations
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-------------------------
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The alias map implementation to use is configured using the `dfs.provided.aliasmap.class` parameter.
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Currently, the following two types of alias maps are supported.
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### InMemoryAliasMap
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This is a LevelDB-based alias map that runs as a separate server in Namenode.
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The alias map itself can be created using the `fs2img` tool using the option
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`-Ddfs.provided.aliasmap.leveldb.path=file:///path/to/leveldb/map/dingos.db -o org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.LevelDBFileRegionAliasMap`
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as in the example above.
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Datanodes contact this alias map using the `org.apache.hadoop.hdfs.server.aliasmap.InMemoryAliasMapProtocol` protocol.
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#### Configuration
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```xml
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<configuration>
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<property>
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<name>dfs.provided.aliasmap.inmemory.batch-size</name>
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<value>500</value>
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<description>
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The batch size when iterating over the database backing the aliasmap
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</description>
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</property>
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<property>
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<name>dfs.provided.aliasmap.inmemory.dnrpc-address</name>
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<value>namenode:rpc-port</value>
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<description>
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The address where the aliasmap server will be running
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</description>
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</property>
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<property>
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<name>dfs.provided.aliasmap.inmemory.leveldb.dir</name>
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<value>file:///path/to/leveldb/map/dingos.db</value>
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<description>
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The directory where the leveldb files will be kept
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</description>
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</property>
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<property>
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<name>dfs.provided.aliasmap.inmemory.enabled</name>
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<value>true</value>
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<description>Enable the inmemory alias map on the NameNode. Defaults to false.</description>
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</property>
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<property>
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<name>dfs.provided.aliasmap.class</name>
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<value>org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.InMemoryLevelDBAliasMapClient</value>
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</property>
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</configuration>
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```
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### TextFileRegionAliasMap
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This alias map implementation stores the mapping from `blockID`s to `FileRegion`s
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in a delimited text file. This format is useful for test environments,
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particularly single-node.
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#### Configuration
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```xml
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<configuration>
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<property>
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<name>dfs.provided.aliasmap.text.delimiter</name>
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<value>,</value>
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<description>
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The delimiter used when the alias map is specified as
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a text file.
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</description>
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</property>
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<property>
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<name>dfs.provided.aliasmap.text.read.file</name>
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<value>file:///path/to/aliasmap/blocks_blocPoolID.csv</value>
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<description>
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The path specifying the alias map as a text file,
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specified as a URI.
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</description>
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</property>
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<property>
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<name>dfs.provided.aliasmap.text.codec</name>
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<value></value>
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<description>
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The codec used to de-compress the alias map. Default value is empty.
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</description>
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</property>
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<property>
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<name>dfs.provided.aliasmap.text.write.dir</name>
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<value>file:///path/to/aliasmap/</value>
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<description>
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The path to which the alias map should be written as a text
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file, specified as a URI.
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</description>
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</property>
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</configuration>
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```
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