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From lium...@apache.org
Subject hadoop git commit: HADOOP-13655. document object store use with fs shell and distcp. Contributed by Steve Loughran
Date Tue, 22 Nov 2016 21:18:32 GMT
Repository: hadoop
Updated Branches:
  refs/heads/trunk 83cc7263a -> beb70fed4


HADOOP-13655. document object store use with fs shell and distcp. Contributed by Steve Loughran

This closes #131


Project: http://git-wip-us.apache.org/repos/asf/hadoop/repo
Commit: http://git-wip-us.apache.org/repos/asf/hadoop/commit/beb70fed
Tree: http://git-wip-us.apache.org/repos/asf/hadoop/tree/beb70fed
Diff: http://git-wip-us.apache.org/repos/asf/hadoop/diff/beb70fed

Branch: refs/heads/trunk
Commit: beb70fed4f15cd4afe8ea23e6068a8344d3557b1
Parents: 83cc726
Author: Mingliang Liu <liuml07@apache.org>
Authored: Mon Nov 21 17:49:05 2016 -0800
Committer: Mingliang Liu <liuml07@apache.org>
Committed: Tue Nov 22 13:12:23 2016 -0800

----------------------------------------------------------------------
 .../src/site/markdown/FileSystemShell.md        | 324 ++++++++++++++++++-
 .../site/markdown/filesystem/introduction.md    |   4 +-
 .../src/site/markdown/DistCp.md.vm              | 131 +++++++-
 3 files changed, 431 insertions(+), 28 deletions(-)
----------------------------------------------------------------------


http://git-wip-us.apache.org/repos/asf/hadoop/blob/beb70fed/hadoop-common-project/hadoop-common/src/site/markdown/FileSystemShell.md
----------------------------------------------------------------------
diff --git a/hadoop-common-project/hadoop-common/src/site/markdown/FileSystemShell.md b/hadoop-common-project/hadoop-common/src/site/markdown/FileSystemShell.md
index c48ff5c..43fc28b 100644
--- a/hadoop-common-project/hadoop-common/src/site/markdown/FileSystemShell.md
+++ b/hadoop-common-project/hadoop-common/src/site/markdown/FileSystemShell.md
@@ -53,10 +53,14 @@ Returns 0 on success and 1 on error.
 cat
 ---
 
-Usage: `hadoop fs -cat URI [URI ...]`
+Usage: `hadoop fs -cat [-ignoreCrc] URI [URI ...]`
 
 Copies source paths to stdout.
 
+Options
+
+* The `-ignoreCrc` option disables checkshum verification.
+
 Example:
 
 * `hadoop fs -cat hdfs://nn1.example.com/file1 hdfs://nn2.example.com/file2`
@@ -116,11 +120,16 @@ copyFromLocal
 
 Usage: `hadoop fs -copyFromLocal <localsrc> URI`
 
-Similar to put command, except that the source is restricted to a local file reference.
+Similar to the `fs -put` command, except that the source is restricted to a local file reference.
 
 Options:
 
-* The -f option will overwrite the destination if it already exists.
+* `-p` : Preserves access and modification times, ownership and the permissions.
+(assuming the permissions can be propagated across filesystems)
+* `-f` : Overwrites the destination if it already exists.
+* `-l` : Allow DataNode to lazily persist the file to disk, Forces a replication
+ factor of 1. This flag will result in reduced durability. Use with care.
+* `-d` : Skip creation of temporary file with the suffix `._COPYING_`.
 
 copyToLocal
 -----------
@@ -300,7 +309,7 @@ Returns 0 on success and -1 on error.
 get
 ---
 
-Usage: `hadoop fs -get [-ignorecrc] [-crc] <src> <localdst> `
+Usage: `hadoop fs -get [-ignorecrc] [-crc] [-p] [-f] <src> <localdst> `
 
 Copy files to the local file system. Files that fail the CRC check may be copied with the
-ignorecrc option. Files and CRCs may be copied using the -crc option.
 
@@ -315,7 +324,11 @@ Returns 0 on success and -1 on error.
 
 Options:
 
-The -f option will overwrite the destination if it already exists.
+* `-p` : Preserves access and modification times, ownership and the permissions.
+(assuming the permissions can be propagated across filesystems)
+* `-f` : Overwrites the destination if it already exists.
+* `-ignorecrc` : Skip CRC checks on the file(s) downloaded.
+* `-crc`: write CRC checksums for the files downloaded.
 
 getfacl
 -------
@@ -483,13 +496,28 @@ Returns 0 on success and -1 on error.
 put
 ---
 
-Usage: `hadoop fs -put <localsrc> ... <dst> `
+Usage: `hadoop fs -put  [-f] [-p] [-l] [-d] [ - | <localsrc1>  .. ]. <dst>`
+
+Copy single src, or multiple srcs from local file system to the destination file system.
+Also reads input from stdin and writes to destination file system if the source is set to
"-"
+
+Copying fails if the file already exists, unless the -f flag is given.
+
+Options:
 
-Copy single src, or multiple srcs from local file system to the destination file system.
Also reads input from stdin and writes to destination file system.
+* `-p` : Preserves access and modification times, ownership and the permissions.
+(assuming the permissions can be propagated across filesystems)
+* `-f` : Overwrites the destination if it already exists.
+* `-l` : Allow DataNode to lazily persist the file to disk, Forces a replication
+ factor of 1. This flag will result in reduced durability. Use with care.
+* `-d` : Skip creation of temporary file with the suffix `._COPYING_`.
+
+
+Examples:
 
 * `hadoop fs -put localfile /user/hadoop/hadoopfile`
-* `hadoop fs -put localfile1 localfile2 /user/hadoop/hadoopdir`
-* `hadoop fs -put localfile hdfs://nn.example.com/hadoop/hadoopfile`
+* `hadoop fs -put -f localfile1 localfile2 /user/hadoop/hadoopdir`
+* `hadoop fs -put -d localfile hdfs://nn.example.com/hadoop/hadoopfile`
 * `hadoop fs -put - hdfs://nn.example.com/hadoop/hadoopfile` Reads the input from stdin.
 
 Exit Code:
@@ -696,7 +724,7 @@ touchz
 
 Usage: `hadoop fs -touchz URI [URI ...]`
 
-Create a file of zero length.
+Create a file of zero length. An error is returned if the file exists with non-zero length.
 
 Example:
 
@@ -729,3 +757,279 @@ usage
 Usage: `hadoop fs -usage command`
 
 Return the help for an individual command.
+
+
+<a name="ObjectStores" />Working with Object Storage
+====================================================
+
+The Hadoop FileSystem shell works with Object Stores such as Amazon S3,
+Azure WASB and OpenStack Swift.
+
+
+
+```bash
+# Create a directory
+hadoop fs -mkdir s3a://bucket/datasets/
+
+# Upload a file from the cluster filesystem
+hadoop fs -put /datasets/example.orc s3a://bucket/datasets/
+
+# touch a file
+hadoop fs -touchz wasb://yourcontainer@youraccount.blob.core.windows.net/touched
+```
+
+Unlike a normal filesystem, renaming files and directories in an object store
+usually takes time proportional to the size of the objects being manipulated.
+As many of the filesystem shell operations
+use renaming as the final stage in operations, skipping that stage
+can avoid long delays.
+
+In particular, the `put` and `copyFromLocal` commands should
+both have the `-d` options set for a direct upload.
+
+
+```bash
+# Upload a file from the cluster filesystem
+hadoop fs -put -d /datasets/example.orc s3a://bucket/datasets/
+
+# Upload a file from under the user's home directory in the local filesystem.
+# Note it is the shell expanding the "~", not the hadoop fs command
+hadoop fs -copyFromLocal -d -f ~/datasets/devices.orc s3a://bucket/datasets/
+
+# create a file from stdin
+# the special "-" source means "use stdin"
+echo "hello" | hadoop fs -put -d -f - wasb://yourcontainer@youraccount.blob.core.windows.net/hello.txt
+
+```
+
+Objects can be downloaded and viewed:
+
+```bash
+# copy a directory to the local filesystem
+hadoop fs -copyToLocal s3a://bucket/datasets/
+
+# copy a file from the object store to the cluster filesystem.
+hadoop fs -get wasb://yourcontainer@youraccount.blob.core.windows.net/hello.txt /examples
+
+# print the object
+hadoop fs -cat wasb://yourcontainer@youraccount.blob.core.windows.net/hello.txt
+
+# print the object, unzipping it if necessary
+hadoop fs -text wasb://yourcontainer@youraccount.blob.core.windows.net/hello.txt
+
+## download log files into a local file
+hadoop fs -getmerge wasb://yourcontainer@youraccount.blob.core.windows.net/logs\* log.txt
+```
+
+Commands which list many files tend to be significantly slower than when
+working with HDFS or other filesystems
+
+```bash
+hadoop fs -count s3a://bucket/
+hadoop fs -du s3a://bucket/
+```
+
+Other slow commands include `find`, `mv`, `cp` and `rm`.
+
+**Find**
+
+This can be very slow on a large store with many directories under the path
+supplied.
+
+```bash
+# enumerate all files in the object store's container.
+hadoop fs -find s3a://bucket/ -print
+
+# remember to escape the wildcards to stop the shell trying to expand them first
+hadoop fs -find s3a://bucket/datasets/ -name \*.txt -print
+```
+
+**Rename**
+
+The time to rename a file depends on its size.
+
+The time to rename a directory depends on the number and size of all files
+beneath that directory.
+
+```bash
+hadoop fs -mv s3a://bucket/datasets s3a://bucket/historical
+```
+
+If the operation is interrupted, the object store will be in an undefined
+state.
+
+**Copy**
+
+```bash
+hadoop fs -cp s3a://bucket/datasets s3a://bucket/historical
+```
+
+The copy operation reads each file and then writes it back to the object store;
+the time to complete depends on the amount of data to copy, and the bandwidth
+in both directions between the local computer and the object store.
+
+**The further the computer is from the object store, the longer the copy takes**
+
+Deleting objects
+----------------
+
+The `rm` command will delete objects and directories full of objects.
+If the object store is *eventually consistent*, `fs ls` commands
+and other accessors may briefly return the details of the now-deleted objects; this
+is an artifact of object stores which cannot be avoided.
+
+If the filesystem client is configured to copy files to a trash directory,
+this will be in the bucket; the `rm` operation will then take time proportional
+to the size of the data. Furthermore, the deleted files will continue to incur
+storage costs.
+
+To avoid this, use the the `-skipTrash` option.
+
+```bash
+hadoop fs -rm -skipTrash s3a://bucket/dataset
+```
+
+Data moved to the `.Trash` directory can be purged using the `expunge` command.
+As this command only works with the default filesystem, it must be configured to
+make the default filesystem the target object store.
+
+```bash
+hadoop fs -expunge -D fs.defaultFS=s3a://bucket/
+```
+
+Overwriting Objects
+----------------
+
+If an object store is *eventually consistent*, then any operation which
+overwrites existing objects may not be immediately visible to all clients/queries.
+That is: later operations which query the same object's status or contents
+may get the previous object. This can sometimes surface within the same client,
+while reading a single object.
+
+Avoid having a sequence of commands which overwrite objects and then immediately
+work on the updated data; there is a risk that the previous data will be used
+instead.
+
+Timestamps
+----------
+
+Timestamps of objects and directories in Object Stores
+may not follow the behavior of files and directories in HDFS.
+
+1. The creation and initial modification times of an object will be the
+time it was created on the object store; this will be at the end of the write process,
+not the beginning.
+1. The timestamp will be taken from the object store infrastructure's clock, not that of
+the client.
+1. If an object is overwritten, the modification time will be updated.
+1. Directories may or may not have valid timestamps. They are unlikely
+to have their modification times updated when an object underneath is updated.
+1. The `atime` access time feature is not supported by any of the object stores
+found in the Apache Hadoop codebase.
+
+Consult the `DistCp` documentation for details on how this may affect the
+`distcp -update` operation.
+
+Security model and operations
+-----------------------------
+
+The security and permissions models of object stores are usually very different
+from those of a Unix-style filesystem; operations which query or manipulate
+permissions are generally unsupported.
+
+Operations to which this applies include: `chgrp`, `chmod`, `chown`,
+`getfacl`, and `setfacl`. The related attribute commands `getfattr` and`setfattr`
+are also usually unavailable.
+
+* Filesystem commands which list permission and user/group details, usually
+simulate these details.
+
+* Operations which try to preserve permissions (example `fs -put -p`)
+do not preserve permissions for this reason. (Special case: `wasb://`, which preserves
+permissions but does not enforce them).
+
+When interacting with read-only object stores, the permissions found in "list"
+and "stat" commands may indicate that the user has write access, when in fact they do not.
+
+Object stores usually have permissions models of their own,
+models can be manipulated through store-specific tooling.
+Be aware that some of the permissions which an object store may provide
+(such as write-only paths, or different permissions on the root path) may
+be incompatible with the Hadoop filesystem clients. These tend to require full
+read and write access to the entire object store bucket/container into which they write data.
+
+As an example of how permissions are mocked, here is a listing of Amazon's public,
+read-only bucket of Landsat images:
+
+```bash
+$ hadoop fs -ls s3a://landsat-pds/
+Found 10 items
+drwxrwxrwx   - mapred          0 2016-09-26 12:16 s3a://landsat-pds/L8
+-rw-rw-rw-   1 mapred      23764 2015-01-28 18:13 s3a://landsat-pds/index.html
+drwxrwxrwx   - mapred          0 2016-09-26 12:16 s3a://landsat-pds/landsat-pds_stats
+-rw-rw-rw-   1 mapred        105 2016-08-19 18:12 s3a://landsat-pds/robots.txt
+-rw-rw-rw-   1 mapred         38 2016-09-26 12:16 s3a://landsat-pds/run_info.json
+drwxrwxrwx   - mapred          0 2016-09-26 12:16 s3a://landsat-pds/runs
+-rw-rw-rw-   1 mapred   27458808 2016-09-26 12:16 s3a://landsat-pds/scene_list.gz
+drwxrwxrwx   - mapred          0 2016-09-26 12:16 s3a://landsat-pds/tarq
+drwxrwxrwx   - mapred          0 2016-09-26 12:16 s3a://landsat-pds/tarq_corrupt
+drwxrwxrwx   - mapred          0 2016-09-26 12:16 s3a://landsat-pds/test
+```
+
+1. All files are listed as having full read/write permissions.
+1. All directories appear to have full `rwx` permissions.
+1. The replication count of all files is "1".
+1. The owner of all files and directories is declared to be the current user (`mapred`).
+1. The timestamp of all directories is actually that of the time the `-ls` operation
+was executed. This is because these directories are not actual objects in the store;
+they are simulated directories based on the existence of objects under their paths.
+
+When an attempt is made to delete one of the files, the operation fails —despite
+the permissions shown by the `ls` command:
+
+```bash
+$ hadoop fs -rm s3a://landsat-pds/scene_list.gz
+rm: s3a://landsat-pds/scene_list.gz: delete on s3a://landsat-pds/scene_list.gz:
+  com.amazonaws.services.s3.model.AmazonS3Exception: Access Denied (Service: Amazon S3;
+  Status Code: 403; Error Code: AccessDenied; Request ID: 1EF98D5957BCAB3D),
+  S3 Extended Request ID: wi3veOXFuFqWBUCJgV3Z+NQVj9gWgZVdXlPU4KBbYMsw/gA+hyhRXcaQ+PogOsDgHh31HlTCebQ=
+```
+
+This demonstrates that the listed permissions cannot
+be taken as evidence of write access; only object manipulation can determine
+this.
+
+Note that the Microsoft Azure WASB filesystem does allow permissions to be set and checked,
+however the permissions are not actually enforced. This feature offers
+the ability for a HDFS directory tree to be backed up with DistCp, with
+its permissions preserved, permissions which may be restored when copying
+the directory back into HDFS. For securing access to the data in the object
+store, however, Azure's [own model and tools must be used](https://azure.microsoft.com/en-us/documentation/articles/storage-security-guide/).
+
+
+Commands of limited value
+---------------------------
+
+Here is the list of shell commands which generally have no effect —and  may
+actually fail.
+
+| command   | limitations |
+|-----------|-------------|
+| `appendToFile` | generally unsupported |
+| `checksum` | the usual checksum is "NONE" |
+| `chgrp` | generally unsupported permissions model; no-op |
+| `chmod` | generally unsupported permissions model; no-op|
+| `chown` | generally unsupported permissions model; no-op |
+| `createSnapshot` | generally unsupported |
+| `deleteSnapshot` | generally unsupported |
+| `df` | default values are normally displayed |
+| `getfacl` | may or may not be supported |
+| `getfattr` | generally supported |
+| `renameSnapshot` | generally unsupported |
+| `setfacl` | generally unsupported permissions model |
+| `setfattr` | generally unsupported permissions model |
+| `setrep`| has no effect |
+| `truncate` | generally unsupported |
+
+Different object store clients *may* support these commands: do consult the
+documentation and test against the target store.

http://git-wip-us.apache.org/repos/asf/hadoop/blob/beb70fed/hadoop-common-project/hadoop-common/src/site/markdown/filesystem/introduction.md
----------------------------------------------------------------------
diff --git a/hadoop-common-project/hadoop-common/src/site/markdown/filesystem/introduction.md
b/hadoop-common-project/hadoop-common/src/site/markdown/filesystem/introduction.md
index 194fa15..f6db557 100644
--- a/hadoop-common-project/hadoop-common/src/site/markdown/filesystem/introduction.md
+++ b/hadoop-common-project/hadoop-common/src/site/markdown/filesystem/introduction.md
@@ -30,10 +30,10 @@ are places where HDFS diverges from the expected behaviour of a POSIX
 filesystem.
 
 The behaviour of other Hadoop filesystems are not as rigorously tested.
-The bundled S3 FileSystem makes Amazon's S3 Object Store ("blobstore")
+The bundled S3N and S3A FileSystem clients make Amazon's S3 Object Store ("blobstore")
 accessible through the FileSystem API. The Swift FileSystem driver provides similar
 functionality for the OpenStack Swift blobstore. The Azure object storage
-FileSystem in branch-1-win talks to Microsoft's Azure equivalent. All of these
+FileSystem talks to Microsoft's Azure equivalent. All of these
 bind to object stores, which do have different behaviors, especially regarding
 consistency guarantees, and atomicity of operations.
 

http://git-wip-us.apache.org/repos/asf/hadoop/blob/beb70fed/hadoop-tools/hadoop-distcp/src/site/markdown/DistCp.md.vm
----------------------------------------------------------------------
diff --git a/hadoop-tools/hadoop-distcp/src/site/markdown/DistCp.md.vm b/hadoop-tools/hadoop-distcp/src/site/markdown/DistCp.md.vm
index 40c6b04..fc26321 100644
--- a/hadoop-tools/hadoop-distcp/src/site/markdown/DistCp.md.vm
+++ b/hadoop-tools/hadoop-distcp/src/site/markdown/DistCp.md.vm
@@ -68,7 +68,7 @@ $H3 Basic Usage
 
   This will expand the namespace under `/foo/bar` on nn1 into a temporary file,
   partition its contents among a set of map tasks, and start a copy on each
-  NodeManager from nn1 to nn2.
+  NodeManager from `nn1` to `nn2`.
 
   One can also specify multiple source directories on the command line:
 
@@ -110,7 +110,7 @@ $H3 Basic Usage
   It's also worth noting that if another client is still writing to a source
   file, the copy will likely fail. Attempting to overwrite a file being written
   at the destination should also fail on HDFS. If a source file is (re)moved
-  before it is copied, the copy will fail with a FileNotFoundException.
+  before it is copied, the copy will fail with a `FileNotFoundException`.
 
   Please refer to the detailed Command Line Reference for information on all
   the options available in DistCp.
@@ -196,20 +196,20 @@ $H3 raw Namespace Extended Attribute Preservation
 
   This section only applies to HDFS.
 
-  If the target and all of the source pathnames are in the /.reserved/raw
+  If the target and all of the source pathnames are in the `/.reserved/raw`
   hierarchy, then 'raw' namespace extended attributes will be preserved.
   'raw' xattrs are used by the system for internal functions such as encryption
   meta data. They are only visible to users when accessed through the
-  /.reserved/raw hierarchy.
+  `/.reserved/raw` hierarchy.
 
   raw xattrs are preserved based solely on whether /.reserved/raw prefixes are
   supplied. The -p (preserve, see below) flag does not impact preservation of
   raw xattrs.
 
   To prevent raw xattrs from being preserved, simply do not use the
-  /.reserved/raw prefix on any of the source and target paths.
+  `/.reserved/raw` prefix on any of the source and target paths.
 
-  If the /.reserved/raw prefix is specified on only a subset of the source and
+  If the `/.reserved/raw `prefix is specified on only a subset of the source and
   target paths, an error will be displayed and a non-0 exit code returned.
 
 Command Line Options
@@ -288,27 +288,27 @@ $H3 Copy-listing Generator
   that need copy into a SequenceFile, for consumption by the DistCp Hadoop
   Job. The main classes in this module include:
 
-  1. CopyListing: The interface that should be implemented by any
+  1. `CopyListing`: The interface that should be implemented by any
      copy-listing-generator implementation. Also provides the factory method by
      which the concrete CopyListing implementation is chosen.
-  2. SimpleCopyListing: An implementation of CopyListing that accepts multiple
+  2. `SimpleCopyListing`: An implementation of `CopyListing` that accepts multiple
      source paths (files/directories), and recursively lists all the individual
      files and directories under each, for copy.
-  3. GlobbedCopyListing: Another implementation of CopyListing that expands
+  3. `GlobbedCopyListing`: Another implementation of `CopyListing` that expands
      wild-cards in the source paths.
-  4. FileBasedCopyListing: An implementation of CopyListing that reads the
+  4. `FileBasedCopyListing`: An implementation of `CopyListing` that reads the
      source-path list from a specified file.
 
   Based on whether a source-file-list is specified in the DistCpOptions, the
   source-listing is generated in one of the following ways:
 
-  1. If there's no source-file-list, the GlobbedCopyListing is used. All
+  1. If there's no source-file-list, the `GlobbedCopyListing` is used. All
      wild-cards are expanded, and all the expansions are forwarded to the
      SimpleCopyListing, which in turn constructs the listing (via recursive
      descent of each path).
-  2. If a source-file-list is specified, the FileBasedCopyListing is used.
+  2. If a source-file-list is specified, the `FileBasedCopyListing` is used.
      Source-paths are read from the specified file, and then forwarded to the
-     GlobbedCopyListing. The listing is then constructed as described above.
+     `GlobbedCopyListing`. The listing is then constructed as described above.
 
   One may customize the method by which the copy-listing is constructed by
   providing a custom implementation of the CopyListing interface. The behaviour
@@ -343,13 +343,13 @@ $H3 InputFormats and MapReduce Components
     implementation keeps the setup-time low.
 
   * **DynamicInputFormat and DynamicRecordReader:**
-    The DynamicInputFormat implements org.apache.hadoop.mapreduce.InputFormat,
+    The DynamicInputFormat implements `org.apache.hadoop.mapreduce.InputFormat`,
     and is new to DistCp. The listing-file is split into several "chunk-files",
     the exact number of chunk-files being a multiple of the number of maps
     requested for in the Hadoop Job. Each map task is "assigned" one of the
     chunk-files (by renaming the chunk to the task's id), before the Job is
     launched.
-    Paths are read from each chunk using the DynamicRecordReader, and
+    Paths are read from each chunk using the `DynamicRecordReader`, and
     processed in the CopyMapper. After all the paths in a chunk are processed,
     the current chunk is deleted and a new chunk is acquired. The process
     continues until no more chunks are available.
@@ -403,7 +403,7 @@ $H3 Map sizing
   slower nodes. While this distribution isn't uniform, it is fair with regard
   to each mapper's capacity.
 
-  The dynamic-strategy is implemented by the DynamicInputFormat. It provides
+  The dynamic-strategy is implemented by the `DynamicInputFormat`. It provides
   superior performance under most conditions.
 
   Tuning the number of maps to the size of the source and destination clusters,
@@ -432,6 +432,105 @@ $H3 MapReduce and other side-effects
   * If `mapreduce.map.speculative` is set set final and true, the result of the
     copy is undefined.
 
+$H3 DistCp and Object Stores
+
+DistCp works with Object Stores such as Amazon S3, Azure WASB and OpenStack Swift.
+
+Prequisites
+
+1. The JAR containing the object store implementation is on the classpath,
+along with all of its dependencies.
+1. Unless the JAR automatically registers its bundled filesystem clients,
+the configuration may need to be modified to state the class which
+implements the filesystem schema. All of the ASF's own object store clients
+are self-registering.
+1. The relevant object store access credentials must be available in the cluster
+configuration, or be otherwise available in all cluster hosts.
+
+DistCp can be used to upload data
+
+```bash
+hadoop distcp hdfs://nn1:8020/datasets/set1 s3a://bucket/datasets/set1
+```
+
+To download data
+
+```bash
+hadoop distcp s3a://bucket/generated/results hdfs://nn1:8020/results
+```
+
+To copy data between object stores
+
+```bash
+hadoop distcp s3a://bucket/generated/results \
+  wasb://updates@example.blob.core.windows.net
+```
+
+And do copy data within an object store
+
+```bash
+hadoop distcp wasb://updates@example.blob.core.windows.net/current \
+  wasb://updates@example.blob.core.windows.net/old
+```
+
+And to use `-update` to only copy changed files.
+
+```bash
+hadoop distcp -update -numListstatusThreads 20  \
+  swift://history.cluster1/2016 \
+  hdfs://nn1:8020/history/2016
+```
+
+Because object stores are slow to list files, consider setting the `-numListstatusThreads`
option when performing a `-update` operation
+on a large directory tree (the limit is 40 threads).
+
+When `DistCp -update` is used with object stores,
+generally only the modification time and length of the individual files are compared,
+not any checksums. The fact that most object stores do have valid timestamps
+for directories is irrelevant; only the file timestamps are compared.
+However, it is important to have the clock of the client computers close
+to that of the infrastructure, so that timestamps are consistent between
+the client/HDFS cluster and that of the object store. Otherwise, changed files may be
+missed/copied too often.
+
+**Notes**
+
+* The `-atomic` option causes a rename of the temporary data, so significantly
+increases the time to commit work at the end of the operation. Furthermore,
+as Object Stores other than (optionally) `wasb://` do not offer atomic renames of directories
+the `-atomic` operation doesn't actually deliver what is promised. *Avoid*.
+
+* The `-append` option is not supported.
+
+* The `-diff` and `rdiff` options are not supported
+
+* CRC checking will not be performed, irrespective of the value of the `-skipCrc`
+flag.
+
+* All `-p` options, including those to preserve permissions, user and group information,
attributes
+checksums and replication are generally ignored. The `wasb://` connector will
+preserve the information, but not enforce the permissions.
+
+* Some object store connectors offer an option for in-memory buffering of
+output —for example the S3A connector. Using such option while copying
+large files may trigger some form of out of memory event,
+be it a heap overflow or a YARN container termination.
+This is particularly common if the network bandwidth
+between the cluster and the object store is limited (such as when working
+with remote object stores). It is best to disable/avoid such options and
+rely on disk buffering.
+
+* Copy operations within a single object store still take place in the Hadoop cluster
+—even when the object store implements a more efficient COPY operation internally
+
+    That is, an operation such as
+
+      hadoop distcp s3a://bucket/datasets/set1 s3a://bucket/datasets/set2
+
+    Copies each byte down to the Hadoop worker nodes and back to the
+bucket. As well as being slow, it means that charges may be incurred.
+
+
 Frequently Asked Questions
 --------------------------
 


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