[HDFS-1526] Dfs client name for a map/reduce task should have some randomness

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.23.0
• Component/s: hdfs-client
• Hadoop Flags: Incompatible change, Reviewed
• Release Note: Make a client name has this format: DFSClient_applicationid_randomint_threadid, where applicationid = mapred.task.id or else = "NONMAPREDUCE".
• Assignee: Hairong Kuang
• Reporter: Hairong Kuang
• Created: 03/Dec/10 07:13
• Updated: 18/Dec/11 13:34
• Resolved: 14/Dec/10 17:47

Description

Fsck shows one of the files in our dfs cluster is corrupt.

/bin/hadoop fsck aFile -files -blocks -locations
aFile: 4633 bytes, 2 block(s):
aFile: CORRUPT block blk_-4597378336099313975
OK
0. blk_-4597378336099313975_2284630101 len=0 repl=3 [...]
1. blk_5024052590403223424_2284630107 len=4633 repl=3 [...]Status: CORRUPT

On disk, these two blocks are of the same size and the same content. It turns out the writer of the file is from a multiple threaded map task. Each thread may write to the same file. One possible interaction of two threads might make this to happen:
[T1: create aFile] [T2: delete aFile] [T2: create aFile][T1: addBlock 0 to aFile][T2: addBlock1 to aFile]...

Because T1 and T2 have the same client name, which is the map task id, the above interactions could be done without any lease exception, thus eventually leading to a corrupt file. To solve the problem, a mapreduce task's client name could be formed by its task id followed by a random number.