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https://issues.apache.org/jira/browse/CRUNCH-515?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14522369#comment-14522369
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Ben Roling commented on CRUNCH-515:
-----------------------------------
{quote}
I don't see any obvious way for me to trace back from the stray directories to
the jobs that created them to be able to do analytics to identify which jobs
are leaving behind the most stray directories.
{quote}
A teammate mentioned the relatively obvious fact that the tmp paths are encoded
into the job names of the jobs that use them based on the way that Crunch
includes plan information in the job name. As such, I can join the stray
directory list against the job names to try to analyze which types of jobs are
leaving the most stray directories. This doesn't work perfectly for reasons
such as limits on job name length but it gets me some good information. I just
started looking at a set of such data from stray temp directories left
yesterday in our dev environment and all of the jobs I could match to were
successful jobs. I can also get a good idea of which development teams are
responsible for these jobs to follow up on what they might be doing (or not
doing) to cause the strays much more frequently than other jobs.
> Decrease probability of collision on Crunch temp directories
> ------------------------------------------------------------
>
> Key: CRUNCH-515
> URL: https://issues.apache.org/jira/browse/CRUNCH-515
> Project: Crunch
> Issue Type: Improvement
> Components: Core
> Affects Versions: 0.8.4, 0.11.0
> Reporter: Ben Roling
> Assignee: Josh Wills
> Attachments: CRUNCH-515-1.patch
>
>
> I've heard reports of failures of Crunch pipelines at our organization due to
> collision on temp directories.
> Take the following stack trace from an old internal email thread I dug up as
> an example:
> {noformat}
> 2015-04-02 04:45:49,208 INFO
> org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchControlledJob:
> org.apache.hadoop.mapred.FileAlreadyExistsException: Output directory
> /tmp/crunch-686245394/p2/output already exists
> at
> org.apache.hadoop.mapreduce.lib.output.FileOutputFormat.checkOutputSpecs(FileOutputFormat.java:132)
> at org.apache.hadoop.mapred.JobClient$2.run(JobClient.java:1013)
> at org.apache.hadoop.mapred.JobClient$2.run(JobClient.java:974)
> at java.security.AccessController.doPrivileged(Native Method)
> at javax.security.auth.Subject.doAs(Subject.java:394)
> at
> org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1438)
> at
> org.apache.hadoop.mapred.JobClient.submitJobInternal(JobClient.java:974)
> at org.apache.hadoop.mapreduce.Job.submit(Job.java:582)
> at
> org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchControlledJob.submit(CrunchControlledJob.java:340)
> at
> org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchJobControl.startReadyJobs(CrunchJobControl.java:277)
> at
> org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchJobControl.pollJobStatusAndStartNewOnes(CrunchJobControl.java:316)
> at
> org.apache.crunch.impl.mr.exec.MRExecutor.monitorLoop(MRExecutor.java:113)
> at
> org.apache.crunch.impl.mr.exec.MRExecutor.access$000(MRExecutor.java:55)
> at org.apache.crunch.impl.mr.exec.MRExecutor$1.run(MRExecutor.java:84)
> at java.lang.Thread.run(Thread.java:682)
> {noformat}
> What we found in this case is the pre-existing directory was rather old. It
> hung around because we're doing a poor job of cleaning old garbage out of our
> HDFS /tmp directory. We intend to set up a job to delete stuff older than a
> couple of weeks or so out of /tmp but I think the chances of a collision will
> still be high enough that failures like this might still happen on occasion.
> The temp directory Crunch chooses is a random 31-bit value:
> https://github.com/apache/crunch/blob/apache-crunch-0.11.0/crunch-core/src/main/java/org/apache/crunch/impl/dist/DistributedPipeline.java#L326
> I say 31 bit value because it comes from a 32-bit random integer but only
> includes positive values, thereby excluding 1 bit.
> The following blog post shows some probabilities for 32-bit hash collisions,
> which are essentially the same problem:
> http://preshing.com/20110504/hash-collision-probabilities/
> Since we're dealing with 31 bits instead of 32 the probabilities will be
> higher than expressed there for 32 bits. Even with 32 bits the probability
> of collision is 1 in 100 with just 9292 values.
> I have not done any thorough investigation to understand why, but in our
> production environment we have a lot of Crunch jobs and we are leaving
> 200-300 stray Crunch temp directories per day. Depending on how aggressive
> we get with a scheduled job to clean old stuff out of temp we could still
> have a realistic chance of hitting a collision.
> My proposal is to change the random integer component of the temp path to a
> UUID or something similar to make it drastically more unlikely that a
> collision will ever occur regardless of whether or not "/tmp" is ever cleaned
> up.
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