If corrupted map outputs, reducers get stuck fetching forever
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Key: HADOOP-86
URL: http://issues.apache.org/jira/browse/HADOOP-86
Project: Hadoop
Type: Bug
Reporter: [EMAIL PROTECTED]
In our rack, there is a machine that reliably corrupts map output parts. When
reducers try to pickup the map output, Server#Handler checks the checksum,
notices corruption, moves the bad map output part aside and throws a
ChecksumException. Undeterred, the reducer comes back again minutes later only
this time it gets a FileNotFoundException out of Server#Handler (Because the
part was moved aside). And so it goes till the cows come home.
Doug applied a patch that in map output file, when it notices a fatal
exception, it logs a severe error on the TaskTracker#LOG. Then in TT, if a
severe logging has occurred, TT does a soft restart (TT stays up but closes
down all services and then goes through init again). This patch was committed
(after I suggested it was working), only, later, I noticed the severe log flag
is not cleared across TT restart so TT goes into a cycle of continuous
restarts.
A further patch that clears the severe flag was posted to the list. This
improves things but has issues too in that on revival, the TT continues to be
plagued by reducers looking for parts no longer available for a period of ten
minutes or so until the JobTracker gets around to updating them about change in
where to go get map outputs. During this period, the TT gets restarted 5-10
times -- but eventually comes back on line (There may have been too much damage
done during this period of flux making it so the job will fail).
This issue covers implementing a better solution.
Suggestions include having the TT stay down a period to avoid the incoming
reducers or somehow examining the incoming reducer request, checking its list
of tasks to see if it knows anything of the reducers' request and rejecting it
with a non-severe error if not a map of the currently running TT. A little
birdie (named DC) suggests a better soln. is probably an addition to
intertrackerprotocol so either the TT or the reducer updates JT when corrupted
map output.
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