[HDFS-3705] Add the possibility to mark a node as 'low priority' for read in the DFSClient

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Won't Fix
• Affects Version/s: 1.0.3, 2.0.0-alpha, 3.0.0-alpha1
• Component/s: hdfs-client
• Assignee: Unassigned
• Reporter: Nicolas Liochon
• Created: 23/Jul/12 11:10
• Updated: 12/May/16 18:14
• Resolved: 15/Oct/13 09:06

Description

This has been partly discussed in HBASE-6435.

The DFSClient includes a 'bad nodes' management for reads and writes. Sometimes, the client application already know that some deads are dead or likely to be dead.
An example is the 'HBase Write-Ahead-Log': when HBase reads this file, it knows that the HBase regionserver died, and it's very likely that the box died so the datanode on the same box is dead as well. This is actually critical, because:

• it's the hbase recovery that reads these log files
• if we read them it means that we lost a box, so we have 1 dead replica out the the 3.
• for all files read, we have 33% of chance to go to the dead datanode
• as the box just died, we're very likely to get a timeout exception so we're delaying the hbase recovery by 1 minute. For HBase, it means that the data is not available during this minute.

Issue Links

• is related to: HDFS-1599 Umbrella Jira for Improving HBASE support in HDFS
• is required by: HBASE-5843 Improve HBase MTTR - Mean Time To Recover
• relates to: HBASE-6435 Reading WAL files after a recovery leads to time lost in HDFS timeouts when using dead datanodes
• relates to: HDFS-4754 Add an API in the namenode to mark a datanode as stale