[HADOOP-2559] DFS should place one replica per rack

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.17.0
• Release Note: Change DFS block placement to allocate the first replica locally, the second off-rack, and the third intra-rack from the second.
• Assignee: Lohit Vijaya Renu
• Reporter: Runping Qi
• Created: 09/Jan/08 16:02
• Updated: 08/Jul/09 16:42
• Resolved: 17/Mar/08 23:54

Description

Currently, when writing out a block, dfs will place one copy to a local data node, one copy to a rack local node
and another one to a remote node. This leads to a number of undesired properties:

1. The block will be rack-local to two tacks instead of three, reducing the advantage of rack locality based scheduling by 1/3.

2. The Blocks of a file (especiallya large file) are unevenly distributed over the nodes: One third will be on the local node, and two thirds on the nodes on the same rack. This may make some nodes full much faster than others,
increasing the need of rebalancing. Furthermore, this also make some nodes become "hot spots" if those big
files are popular and accessed by many applications.

Issue Links

• is depended upon by: HDFS-325 DFS should not use round robin policy in determing on which volume (file system partition) to allocate for the next block