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From a..@apache.org
Subject [2/2] hadoop git commit: HDFS-7164. Feature documentation for HDFS-6581. (Contributed by Arpit Agarwal)
Date Wed, 17 Jun 2015 02:07:24 GMT
HDFS-7164. Feature documentation for HDFS-6581. (Contributed by Arpit Agarwal)

Project: http://git-wip-us.apache.org/repos/asf/hadoop/repo
Commit: http://git-wip-us.apache.org/repos/asf/hadoop/commit/acc90ca1
Tree: http://git-wip-us.apache.org/repos/asf/hadoop/tree/acc90ca1
Diff: http://git-wip-us.apache.org/repos/asf/hadoop/diff/acc90ca1

Branch: refs/heads/branch-2
Commit: acc90ca1757d11dd9ee6634e9d83523ad550c12f
Parents: 64389f7
Author: Arpit Agarwal <arp@apache.org>
Authored: Tue Jun 16 19:05:44 2015 -0700
Committer: Arpit Agarwal <arp@apache.org>
Committed: Tue Jun 16 19:06:39 2015 -0700

 hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt     |   2 +
 .../site/markdown/CentralizedCacheManagement.md |   2 +
 .../src/site/markdown/MemoryStorage.md          | 130 +++++++++++++++++++
 .../site/resources/images/LazyPersistWrites.png | Bin 0 -> 107161 bytes
 hadoop-project/src/site/site.xml                |   1 +
 5 files changed, 135 insertions(+)

diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
index 6d9a4b8..6550759 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
@@ -295,6 +295,8 @@ Release 2.8.0 - UNRELEASED
     HDFS-8606. Cleanup DFSOutputStream by removing unwanted changes
     from HDFS-8386. (Rakesh R via szetszwo)
+    HDFS-7164. Feature documentation for HDFS-6581. (Arpit Agarwal)
     HDFS-8026. Trace FSOutputSummer#writeChecksumChunks rather than

diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/CentralizedCacheManagement.md
index b4f08c8..72c125d 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/CentralizedCacheManagement.md
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/CentralizedCacheManagement.md
@@ -233,6 +233,8 @@ Be sure to configure the following:
     This determines the maximum amount of memory a DataNode will use for caching. On Unix-like
systems, the "locked-in-memory size" ulimit (`ulimit -l`) of the DataNode user also needs
to be increased to match this parameter (see below section on [OS Limits](#OS_Limits)). When
setting this value, please remember that you will need space in memory for other things as
well, such as the DataNode and application JVM heaps and the operating system page cache.
+    This setting is shared with the [Lazy Persist Writes feature](./MemoryStorage.html).
The Data Node will ensure that the combined memory used by Lazy Persist Writes and Centralized
Cache Management does not exceed the amount configured in `dfs.datanode.max.locked.memory`.
 #### Optional
 The following properties are not required, but may be specified for tuning:

diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/MemoryStorage.md b/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/MemoryStorage.md
new file mode 100644
index 0000000..1ed4b5a
--- /dev/null
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/MemoryStorage.md
@@ -0,0 +1,130 @@
+  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,
+  See the License for the specific language governing permissions and
+  limitations under the License. See accompanying LICENSE file.
+Memory Storage Support in HDFS
+* [Introduction](#Introduction)
+* [Administrator Configuration](#Administrator_Configuration)
+    * [Limit RAM used for replicas in Memory](#Limit_RAM_used_for_replicas_in_Memory)
+    * [Setup RAM Disks on Data Nodes](#Setup_RAM_Disks_on_Data_Nodes)
+        * [Choosing `tmpfs` \(vs `ramfs`\)](#Choosing_`tmpfs`_\(vs_`ramfs`\))
+        * [Mount RAM Disks](#Mount_RAM_Disks)
+        * [Tag `tmpfs` volume with the RAM\_DISK Storage Type](#Tag_`tmpfs`_volume_with_the_RAM\_DISK_Storage_Type)
+        * [Ensure Storage Policies are enabled](#Ensure_Storage_Policies_are_enabled)
+* [Application Usage](#Application_Usage)
+    * [Use the LAZY\_PERSIST Storage Policy](#Use_the_LAZY\_PERSIST_Storage_Policy)
+        * [Invoke `hdfs storagepolicies` command for directories](#Invoke_hdfs_storagepolicies_command_for_directories)
+        * [Call `setStoragePolicy` method for directories](#Call_`setStoragePolicy`_method_for_directories)
+        * [Pass `LAZY_PERSIST` `CreateFlag` for new files](#Pass_`LAZY_PERSIST`_`CreateFlag`_for_new_files)
+HDFS supports writing to off-heap memory managed by the Data Nodes. The Data Nodes will flush
in-memory data to disk asynchronously thus removing expensive disk IO and checksum computations
from the performance-sensitive IO path, hence we call such writes *Lazy Persist* writes. HDFS
provides best-effort persistence guarantees for Lazy Persist Writes. Rare data loss is possible
in the event of a node restart before replicas are persisted to disk. Applications can choose
to use Lazy Persist Writes to trade off some durability guarantees in favor of reduced latency.
+This feature is available starting with Apache Hadoop 2.6.0 and was developed under Jira
+![Lazy Persist Writes](images/LazyPersistWrites.png)
+The target use cases are applications that would benefit from writing relatively low amounts
of data (from a few GB up to tens of GBs depending on available memory) with low latency.
Memory storage is for applications that run within the cluster and collocated with HDFS Data
Nodes. We have observed that the latency overhead from network replication negates the benefits
of writing to memory.
+Applications that use Lazy Persist Writes will continue to work by falling back to DISK storage
if memory is insufficient or unconfigured.
+Administrator Configuration
+This section enumerates the administrative steps required before applications can start using
the feature in a cluster.
+## Limit RAM used for replicas in Memory
+First decide the amount of memory to be dedicated for replicas stored in memory. Set `dfs.datanode.max.locked.memory`
accordingly in `hdfs-site.xml`. This is the same setting used by the [Centralized Cache Management](./CentralizedCacheManagement.html)
feature. The Data Node will ensure that the combined memory used by Lazy Persist Writes and
Centralized Cache Management does not exceed the amount configured in `dfs.datanode.max.locked.memory`.
+E.g. To reserve 32 GB for in-memory replicas
+        <property>
+          <name>dfs.datanode.max.locked.memory</name>
+          <value>34359738368</value>
+        </property>
+This memory is not allocated by the Data Node on startup.
+On Unix-like systems, the "locked-in-memory size" ulimit (`ulimit -l`) of the Data Node user
also needs to be increased to match this parameter (see the related section on [OS Limits](./CentralizedCacheManagement.html#OS_Limits)).
When setting this value, please remember that you will need space in memory for other things
as well, such as the Data Node and application JVM heaps and the operating system page cache.
You will also need memory for YARN containers if there is a YARN Node Manager process running
on the same node as the Data Node.
+## Setup RAM Disks on Data Nodes
+Initialize a RAM disk on each Data Node. The choice of RAM Disk allows better data persistence
across Data Node process restarts. The following setup will work on most Linux distributions.
Using RAM disks on other platforms is not currently supported.
+### Choosing `tmpfs` \(vs `ramfs`\)
+Linux supports using two kinds of RAM disks - `tmpfs` and `ramfs`. The size of `tmpfs` is
limited by the Linux kernel while `ramfs` grows to fill all available system memory. There
is a downside to `tmpfs` since its contents can be swapped to disk under memory pressure.
However many performance-sensitive deployments run with swapping disabled so we do not expect
this to be an issue in practice.
+HDFS currently supports using `tmpfs` partitions. Support for adding `ramfs` is in progress
(See [HDFS-8584](https://issues.apache.org/jira/browse/HDFS-8584)).
+### Mount RAM Disks
+Mount the RAM Disk partition with the Unix `mount` command. E.g. to mount a 32 GB `tmpfs`
partition under `/mnt/dn-tmpfs/`
+        sudo mount -t tmpfs -o size=32g tmpfs /mnt/dn-tmpfs/
+It is recommended you create an entry in the `/etc/fstab` so the RAM Disk is recreated automatically
on node restarts. Another option is to use a sub-directory under `/dev/shm` which is a `tmpfs`
mount available by default on most Linux distributions. Ensure that the size of the mount
is greater than or equal to your `dfs.datanode.max.locked.memory` setting else override it
in `/etc/fstab`. Using more than one `tmpfs` partition per Data Node for Lazy Persist Writes
is not recommended.
+### Tag `tmpfs` volume with the RAM\_DISK Storage Type
+Tag the `tmpfs` directory with the RAM_DISK storage type via the `dfs.datanode.data.dir`
configuration setting in `hdfs-site.xml`. E.g. On a Data Node with three hard disk volumes
`/grid/0`, `/grid/1` and `/grid/2` and a `tmpfs` mount `/mnt/dn-tmpfs`, `dfs.datanode.data.dir`
must be set as follows:
+        <property>
+          <name>dfs.datanode.data.dir</name>
+          <value>/grid/0,/grid/1,/grid/2,[RAM_DISK]/mnt/dn-tmpfs</value>
+        </property>
+This step is crucial. Without the RAM_DISK tag, HDFS will treat the `tmpfs` volume as non-volatile
storage and data will not be saved to persistent storage. You will lose data on node restart.
+### Ensure Storage Policies are enabled
+Ensure that the global setting to turn on Storage Policies is enabled [as documented here](ArchivalStorage.html#Configuration).
This setting is on by default.
+Application Usage
+## Use the LAZY\_PERSIST Storage Policy
+Applications indicate that HDFS can use Lazy Persist Writes for a file with the `LAZY_PERSIST`
storage policy. Administrative privileges are *not* required to set the policy and it can
be set in one of three ways.
+### Invoke `hdfs storagepolicies` command for directories
+Setting the policy on a directory causes it to take effect for all new files in the directory.
The `hdfs storagepolicies` command can be used to set the policy as described in the [Storage
Policies documentation](ArchivalStorage.html#Storage_Policy_Commands).
+        hdfs storagepolicies -setStoragePolicy -path <path> -policy LAZY_PERSIST
+### Call `setStoragePolicy` method for directories
+Starting with Apache Hadoop 2.8.0, an application can programmatically set the Storage Policy
with `FileSystem.setStoragePolicy`. E.g.
+        fs.setStoragePolicy(path, "LAZY_PERSIST");
+### Pass `LAZY_PERSIST` `CreateFlag` for new files
+An application can pass `CreateFlag#LAZY_PERSIST` when creating a new file with `FileSystem#create`
API. E.g.
+        FSDataOutputStream fos =
+            fs.create(
+                path,
+                FsPermission.getFileDefault(),
+                EnumSet.of(CreateFlag.CREATE, CreateFlag.LAZY_PERSIST),
+                bufferLength,
+                replicationFactor,
+                blockSize,
+                null);

diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/site/resources/images/LazyPersistWrites.png
new file mode 100644
index 0000000..b2bde93
Binary files /dev/null and b/hadoop-hdfs-project/hadoop-hdfs/src/site/resources/images/LazyPersistWrites.png

diff --git a/hadoop-project/src/site/site.xml b/hadoop-project/src/site/site.xml
index c39f62c..e2a1d7f 100644
--- a/hadoop-project/src/site/site.xml
+++ b/hadoop-project/src/site/site.xml
@@ -96,6 +96,7 @@
       <item name="Transparent Encryption" href="hadoop-project-dist/hadoop-hdfs/TransparentEncryption.html"/>
       <item name="Multihoming" href="hadoop-project-dist/hadoop-hdfs/HdfsMultihoming.html"/>
       <item name="Storage Policies" href="hadoop-project-dist/hadoop-hdfs/ArchivalStorage.html"/>
+      <item name="Memory Storage Support" href="hadoop-project-dist/hadoop-hdfs/MemoryStorage.html"/>
     <menu name="MapReduce" inherit="top">

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