Increasing Hadoop’s efficiency is an important aspect of continuing its growth. As a result, researchers from the University of Illinois at Urbana-Champaign and Yahoo conducted a unique study on how MapReduce was accessing files in various Hadoop clusters.
The team behind the research says that from what they’ve been able to see, this effort represents the first examination of how MapReduce workloads interact with the storage layer.
To get the lay of the storage land, they monitored two Hadoop clusters over six months: the 4100+ node PROD cluster, whose jobs run at regular intervals (daily, weekly, etc.) and the 1900+ node R&D cluster, which focuses on research and development: testing jobs for future use on the PROD.
Determining popularity, according to the study, is remarkably difficult. The amount of files is changing constantly as files are added and deleted. As such, dividing a particular file or a group of files’ access by the amount of the files in the namespace produces numbers that are of little mathematical value.
Instead, they flipped the question, asking how many files rarely get queried. According to the report, “80 − 90% of the files are accessed no more than 10 times during the full 6-month period.” There exist so many unpopular files due to the large amount of files that get accessed and then deleted, termed ‘short-lived files’ by the report.
The report also found that the vast majority of the files that were being accessed were relatively young. “What percentage of accesses target files that are at most one week old? The answer, is surprisingly close for both clusters: 90.31% (PROD) and 86.87% (R&D). To provide some perspective, a media server study found that the ﬁrst ﬁve weeks of a ﬁle’s existence account for 70 − 80% of their accesses.”
What is surprising about that closeness is that the R&D system may be expected to run its test jobs on older, less relevant data.
A remarkable amount of jobs (29-30%) used files that were less than two minutes old. However, the report suspects that has to do with the MapReduce job duration. “During the same 6-month period, 34.75% − 57.46% (PROD and R&D) of the successful jobs had a total running time of 1 minute or less (including the time waiting on the scheduler queue).”
Either way, this heavy reliance suggests that an emphasis on speed should be placed on the newer files in future designs. “Their high file churn and skewed access towards young files, among others, should be further studied and modeled to enable designers of next generation file systems to optimize their designs to best meet the requirements of these emerging workloads.”