Kannan Rajah created HADOOP-11905:
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Summary: Abstraction for LocalDirAllocator
Key: HADOOP-11905
URL: https://issues.apache.org/jira/browse/HADOOP-11905
Project: Hadoop Common
Issue Type: Bug
Components: fs
Affects Versions: 2.5.2
Reporter: Kannan Rajah
Assignee: Kannan Rajah
Fix For: 2.7.1
There are 2 abstractions used to write data to local disk.
LocalDirAllocator: Allocate paths from a set of configured local directories.
LocalFileSystem/RawLocalFileSystem: Read/write using java.io.* and java.nio.*
In the current implementation, local disk is managed by guest OS and not HDFS.
The proposal is to provide a new abstraction that encapsulates the above 2
abstractions and hides who manages the local disks. This enables us to provide
an alternate implementation where a DFS can manage the local disks and it can
be accessed using HDFS APIs. This means the DFS maintains a namespace for node
local directories and can create paths that are guaranteed to be present on a
specific node.
Here is an example use case for Shuffle: When a mapper writes intermediate data
using this new implementation, it will continue write to local disk. When a
reducer needs to access data from a remote node, it can use HDFS APIs with a
path that points to that node’s local namespace instead of having to use HTTP
server to transfer the data across nodes.
New Abstractions
1. LocalDiskPathAllocator
Interface to get file/directory paths from the local disk namespace.
This contains all the APIs that are currently supported by LocalDirAllocator.
So we just need to change LocalDirAllocator to implement this new interface.
2. LocalDiskUtil
Helper class to get a handle to LocalDiskPathAllocator and the FileSystem
that is used to manage those paths.
By default, it will return LocalDirAllocator and LocalFileSystem.
A supporting DFS can return DFSLocalDirAllocator and an instance of DFS.
3. DFSLocalDirAllocator
This is a generic implementation. An allocator is created for a specific node.
It uses Configuration object to get user configured base directory and appends
the node hostname to it. Hence the returned paths are within the node local
namespace.
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