Details
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Bug
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Status: Patch Available
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Major
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Resolution: Unresolved
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2.8.3, 3.0.0
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None
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None
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None
Description
In a Hadoop cluster, number of nodes on a rack could be different. For example, we have 50 Datanodes in all and 15 datanodes per rack, it would remain 5 nodes on the last rack. In this situation, we find that storage usage on the last 5 nodes would be much higher than other nodes.
With the default blockplacement policy, for each block, the first replication has the same probability to write to each datanode, but the probability for the 2nd/3rd replication to write to the last 5 nodes would much higher than to other nodes.
Consider we write 50 blocks to such 50 datanodes. The first rep of 100 block would distirbuted to 50 node equally. The 2rd rep of blocks which the 1st rep is on rack1(15 reps) would send equally to other 35 nodes and each nodes receive 0.428 rep. So does blocks on rack2 and rack3. As a result, node on rack4(5 nodes) would receive 1.29 replications in all, while other node would receive 0.97 reps.
| - | Rack1(15 nodes) | Rack2(15 nodes) | Rack3(15 nodes) | Rack4(5 nodes) |
|---|---|---|---|---|
| From rack1 | - | 15/35=0.43 | 0.43 | 0.43 |
| From rack2 | 0.43 | - | 0.43 | 0.43 |
| From rack3 | 0.43 | 0.43 | - | 0.43 |
| From rack4 | 5/45=0.11 | 0.11 | 0.11 | - |
| Total | 0.97 | 0.97 | 0.97 | 1.29 |