HDFS-12874. Documentation for provided storage. Contributed by Virajith Jalaparti

hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml

@@ -4643,7 +4643,7 @@
     <description>
       The class that is used to specify the input format of the blocks on
       provided storages. The default is
-      org.apache.hadoop.hdfs.server.common.TextFileRegionAliasMap which uses
+      org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.TextFileRegionAliasMap which uses
       file regions to describe blocks. The file regions are specified as a
       delimited text file. Each file region is a 6-tuple containing the
       block id, remote file path, offset into file, length of block, the

hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HdfsProvidedStorage.md

@@ -0,0 +1,247 @@
+<!---
+  Licensed 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. See accompanying LICENSE file.
+-->
+
+HDFS Provided Storage
+=====================
+
+Provided storage allows data *stored outside HDFS* to be mapped to and addressed
+from HDFS. It builds on [heterogeneous storage](./ArchivalStorage.html) by
+introducing a new storage type, `PROVIDED`, to the set of media in a datanode.
+Clients accessing data in
+`PROVIDED` storages can cache replicas in local media, enforce HDFS invariants
+(e.g., security, quotas), and address more data than the cluster could persist
+in the storage attached to DataNodes. This architecture is particularly useful
+in scenarios where HDFS clusters are ephemeral (e.g., cloud scenarios), and/or
+require to read data that lives in other storage systems (e.g., blob stores).
+
+Provided storage is an experimental feature in HDFS.
+
+<!-- MACRO{toc|fromDepth=0|toDepth=3} -->
+
+Introduction
+------------
+
+As of this writing, support for mounting external storage as `PROVIDED` blocks
+is limited to creating a *read-only image* of a remote namespace that implements the
+`org.apache.hadoop.fs.FileSystem` interface, and starting a NameNode
+to serve the image. Specifically, reads from a snapshot of a remote namespace are
+supported. Adding a remote namespace to an existing/running namenode, refreshing the
+remote snapshot, unmounting, and writes are not available in this release. One
+can use [ViewFs](./ViewFs.html) and [RBF](HDFSRouterFederation.html) to
+integrate namespaces with `PROVIDED` storage into an existing deployment.
+
+Creating HDFS Clusters with `PROVIDED` Storage
+----------------------------------------------
+
+One can create snapshots of the remote namespace using the `fs2img` tool. Given
+a path to a remote `FileSystem`, the tool creates an _image_ mirroring the
+namespace and an _alias map_ that maps blockIDs in the generated image to a
+`FileRegion` in the remote filesystem. A `FileRegion` contains sufficient information to
+address a fixed sequence of bytes in the remote `FileSystem` (e.g., file, offset, length)
+and a nonce to verify that the region is unchanged since the image was generated.
+
+After the NameNode image and alias map are created, the NameNode and DataNodes
+must be configured to consistently reference this address space. When a DataNode
+registers with an attached, `PROVIDED` storage, the NameNode considers all the
+external blocks to be addressable through that DataNode, and may begin to direct
+clients to it. Symmetrically, the DataNode must be able to map every block in
+the `PROVIDED` storage to remote data.
+
+Deployment details vary depending on the configured alias map implementation.
+
+### `PROVIDED` Configuration
+
+Each NameNode supports one alias map. When `PROVIDED` storage is enabled,
+the storage ID configured on the NameNode and DataNodes must match.
+All other details are internal to the alias map implementation.
+
+The configuration to enable `PROVIDED` storage is as follows.
+The configuration options available for the alias map implementations are
+available below.
+
+```xml
+<configuration>
+
+  <property>
+    <name>dfs.namenode.provided.enabled</name>
+    <value>true</value>
+    <description>Enabled provided storage on the Namenode</description>
+  </property>
+
+  <property>
+     <name>dfs.datanode.data.dir</name>
+     <value>[DISK]/local/path/to/blocks/, [PROVIDED]remoteFS://remoteFS-authority/path/to/data/</value>
+  </property>
+
+  <property>
+      <name>dfs.provided.storage.id</name>
+      <value>DS-PROVIDED</value>
+      <description>The storage ID used for provided storages in the cluster.</description>
+  </property>
+
+  <property>
+    <name>dfs.provided.aliasmap.class</name>
+    <value>org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.TextFileRegionAliasMap</value>
+  </property>
+
+</configuration>
+```
+
+### fs2img tool
+
+The `fs2img` tool "walks" over a remote namespace by recursively enumerating
+children of a remote URI to produce an FSImage. Some attributes can be
+controlled by plugins, such as owner/group mappings from the remote filesystem
+to HDFS and the mapping of files to HDFS blocks.
+
+The various options available in running the tool are:
+
+| Option                  | Property                    | Default           | Description |
+|:------------------------|:--------------------------- |:----------------- |:---- |
+| `-o`, `--outdir`        | dfs.namenode.name.dir       | file://${hadoop.tmp.dir}/dfs/name | Output directory |
+| `-b`, `--blockclass`    | dfs.provided.aliasmap.class | NullBlocksMap     | Block output class |
+| `-u`, `--ugiclass`      | hdfs.image.writer.ugi.class | SingleUGIResolver | UGI resolver class |
+| `-i`, `--blockidclass`  | hdfs.image.writer.blockresolver.class | FixedBlockResolver | Block resolver class |
+| `-c`, `--cachedirs`     | hdfs.image.writer.cache.entries | 100           | Max active dirents |
+| `-cid`, `--clusterID`   |                             |                   | Cluster ID |
+| `-bpid`, `--blockPoolID`|                             |                   | Block pool ID |
+
+#### Examples
+
+Assign all files to be owned by "rmarathe", write to gzip compressed text:
+```
+hadoop org.apache.hadoop.hdfs.server.namenode.FileSystemImage \
+  -Dhdfs.image.writer.ugi.single.user=rmarathe \
+  -Ddfs.provided.aliasmap.text.codec=gzip \
+  -Ddfs.provided.aliasmap.text.write.dir=file:///tmp/
+  -b org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.TextFileRegionAliasMap \
+  -u org.apache.hadoop.hdfs.server.namenode.SingleUGIResolver \
+  -o file:///tmp/name \
+  hdfs://afreast/projects/ydau/onan
+```
+
+Assign ownership based on a custom `UGIResolver`, in LevelDB:
+```
+hadoop org.apache.hadoop.hdfs.server.namenode.FileSystemImage \
+  -Ddfs.provided.aliasmap.leveldb.path=file:///path/to/leveldb/map/dingos.db \
+  -b org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.LevelDBFileRegionAliasMap \
+  -o file:///tmp/name \
+  -u CustomResolver \
+  hdfs://enfield/projects/ywqmd/incandenza
+```
+
+
+Alias Map Implementations
+-------------------------
+
+The alias map implementation to use is configured using the `dfs.provided.aliasmap.class` parameter.
+Currently, the following two types of alias maps are supported.
+
+### InMemoryAliasMap
+
+This is a LevelDB-based alias map that runs as a separate server in Namenode.
+The alias map itself can be created using the `fs2img` tool using the option
+`-Ddfs.provided.aliasmap.leveldb.path=file:///path/to/leveldb/map/dingos.db -o org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.LevelDBFileRegionAliasMap`
+as in the example above.
+
+Datanodes contact this alias map using the `org.apache.hadoop.hdfs.server.aliasmap.InMemoryAliasMapProtocol` protocol.
+
+#### Configuration
+
+
+```xml
+<configuration>
+  <property>
+    <name>dfs.provided.aliasmap.inmemory.batch-size</name>
+    <value>500</value>
+    <description>
+      The batch size when iterating over the database backing the aliasmap
+    </description>
+  </property>
+
+  <property>
+    <name>dfs.provided.aliasmap.inmemory.dnrpc-address</name>
+    <value>namenode:rpc-port</value>
+    <description>
+      The address where the aliasmap server will be running
+    </description>
+  </property>
+
+  <property>
+    <name>dfs.provided.aliasmap.inmemory.leveldb.dir</name>
+    <value>file:///path/to/leveldb/map/dingos.db</value>
+    <description>
+      The directory where the leveldb files will be kept
+    </description>
+  </property>
+
+  <property>
+    <name>dfs.provided.aliasmap.inmemory.enabled</name>
+    <value>true</value>
+    <description>Enable the inmemory alias map on the NameNode. Defaults to false.</description>
+  </property>
+
+  <property>
+    <name>dfs.provided.aliasmap.class</name>
+    <value>org.apache.hadoop.hdfs.server.common.blockaliasmap.impl.InMemoryLevelDBAliasMapClient</value>
+  </property>
+</configuration>
+```
+
+### TextFileRegionAliasMap
+
+This alias map implementation stores the mapping from `blockID`s to `FileRegion`s
+in a delimited text file. This format is useful for test environments,
+particularly single-node.
+
+#### Configuration
+```xml
+<configuration>
+  <property>
+    <name>dfs.provided.aliasmap.text.delimiter</name>
+    <value>,</value>
+    <description>
+        The delimiter used when the alias map is specified as
+        a text file.
+    </description>
+  </property>
+
+  <property>
+    <name>dfs.provided.aliasmap.text.read.file</name>
+    <value>file:///path/to/aliasmap/blocks_blocPoolID.csv</value>
+    <description>
+        The path specifying the alias map as a text file,
+        specified as a URI.
+    </description>
+  </property>
+
+  <property>
+    <name>dfs.provided.aliasmap.text.codec</name>
+    <value></value>
+    <description>
+        The codec used to de-compress the alias map. Default value is empty.
+    </description>
+  </property>
+
+  <property>
+    <name>dfs.provided.aliasmap.text.write.dir</name>
+    <value>file:///path/to/aliasmap/</value>
+    <description>
+        The path to which the alias map should be written as a text
+        file, specified as a URI.
+    </description>
+  </property>
+</configuration>
+```
+