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Dr. Hadoop: an infinite scalable metadata management for Hadoop—How the baby elephant becomes immortal
Dipayan DEV,Ripon PATGIRI
Front. Inform. Technol. Electron. Eng.    2016, 17 (1): 15-31.   DOI: 10.1631/FITEE.1500015
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In this Exa byte scale era, data increases at an exponential rate. This is in turn generating a massive amount of metadata in the file system. Hadoop is the most widely used framework to deal with big data. Due to this growth of huge amount of metadata, however, the efficiency of Hadoop is questioned numerous times by many researchers. Therefore, it is essential to create an efficient and scalable metadata management for Hadoop. Hash-based mapping and subtree partitioning are suitable in distributed metadata management schemes. Subtree partitioning does not uniformly distribute workload among the metadata servers, and metadata needs to be migrated to keep the load roughly balanced. Hash-based mapping suffers from a constraint on the locality of metadata, though it uniformly distributes the load among NameNodes, which are the metadata servers of Hadoop. In this paper, we present a circular metadata management mechanism named dynamic circular metadata splitting (DCMS). DCMS preserves metadata locality using consistent hashing and locality-preserving hashing, keeps replicated metadata for excellent reliability, and dynamically distributes metadata among the NameNodes to keep load balancing. NameNode is a centralized heart of the Hadoop. Keeping the directory tree of all files, failure of which causes the single point of failure (SPOF). DCMS removes Hadoop’s SPOF and provides an efficient and scalable metadata management. The new framework is named ‘Dr. Hadoop’ after the name of the authors.




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Fig. 8 Linear growth of metadata for Hadoop and Dr. Hadoop load per NameNode: (a) Microsoft Windows trace; (b) Yahoo trace
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To better understand the accuracy of the simulator, in Fig. 8a, a comparison between simulator results and real-time results is shown.
The scalability of Dr. Hadoop is analyzed with the two real traces as shown in Table 2. The growth of metadata (namespace) is studied with the increasing data uploaded to the cluster in HDFS. These observations are tabulated in Tables 3 , 4 , and 5 for Hadoop and Dr. Hadoop with 10 GB data uploaded on each attempt. The metadata size of Table 5 is 3 times its original size for Yahoo and Microsoft respectively because of the replication. Figs. 8a and 8b represent the scalability of Hadoop and Dr. Hadoop in terms of load (MB) /NameNode for both datasets. The graphs show a linear increment in the metadata size of Hadoop and Dr. Hadoop. In traditional Hadoop, with the increase in data size in the DataNodes, the metadata is likely to grow to the upper bound of the main memory of a single NameNode. So, the maximum limit of data size that DataNodes can afford is limited to the size of available memory on the single NameNode server. In Dr. Hadoop, DCMS provides a cluster of NameNodes, which reduces the metadata load rate per NameNode for the cluster. This results in enormous increase in the storage capacity of Dr. Hadoop.
In Fig. 8a, we have also compared the value obtained by the simulator with that by real-time analysis. As the result suggests, the percentage of mismatch is very little and lines for Hadoop and Hadoop_RT are almost proximate. This validates the accuracy of the simulator.
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