Sizing NameNode Heap Memory

Each workload has a unique byte-distribution profile. Some workloads can use the default JVM settings for heap memory and garbage collection, but others require tuning. This topic provides guidance on sizing your NameNode JVM if the dynamic heap settings cause a bottleneck.

All Hadoop processes run on a Java Virtual Machine (JVM). The number of JVMs depend on your deployment mode:

  • Local (or standalone) mode - There are no daemons and everything runs on a single JVM.
  • Pseudo-distributed mode - Each daemon (such as the NameNode daemon) runs on its own JVM on a single host.
  • Distributed mode - Each daemon runs on its own JVM across a cluster of hosts.

The standard NameNode configuration is one active (and primary) NameNode for the entire namespace and one Secondary NameNode for checkpoints (but not failover). A high- availability configuration replaces the Secondary NameNode with a Standby NameNode that prevents a single point of failure. Each NameNode uses its own JVM.

Environment Variables

HADOOP_HEAPSIZE sets the JVM heap size for all Hadoop project servers such as HDFS, YARN, and MapReduce. HADOOP_HEAPSIZE is an integer passed to the JVM as the maximum memory (Xmx) argument. For example: 
HADOOP_HEAPSIZE=1024
HADOOP_NAMENODE_OPTS is specific to the NameNode and sets all JVM flags, which must be specified. HADOOP_NAMENODE_OPTS overrides the HADOOP_HEAPSIZE Xmx value for the NameNode. For example: 
HADOOP_NAMENODE_OPTS=-Xms1024m -Xmx1024m -XX:+UseParNewGC -XX:+UseConcMarkSweepGC -XX:CMSInitiatingOccupancyFraction=70 -XX:+CMSParallelRemarkEnabled -XX:+PrintTenuringDistribution -XX:OnOutOfMemoryError={{AGENT_COMMON_DIR}}/killparent.sh

Both HADOOP_NAMENODE_OPTS and HADOOP_HEAPSIZE are stored in /etc/hadoop/conf/hadoop-env.sh.

Monitoring Heap Memory Usage

You can monitor your heap memory usage several ways:

  • Cloudera Manager: Look at the NameNode chart for heap memory usage. If you need to build the chart from scratch, run: 
    select jvm_max_memory_mb, jvm_heap_used_mb where roleType="NameNode"
  • NameNode Web UI: Scroll down to the Summary and look for "Heap Memory used."
  • Command line: Generate a heap dump.

Files and Blocks

In HDFS, data and metadata are decoupled. Data files are split into block files that are stored, and replicated, on DataNodes across the cluster. The filesystem namespace tree and associated metadata are stored on the NameNode.

Namespace objects are file inodes and blocks that point to block files on the DataNodes. These namespace objects are stored as a file system image (fsimage) in the NameNode's memory and also persist locally. Updates to the metadata are written to an edit log. When the NameNode starts, or when a checkpoint is taken, the edits are applied, the log is cleared, and a new fsimage is created.

On average, each file consumes 1.5 blocks of storage. That is, the average file is split into two block files—one that consumes the entire allocated block size and a second that consumes half of that. On the NameNode, this same average file requires three namespace objects—one file inode and two blocks.

Disk Space versus Namespace

The CDH default block size (dfs.blocksize) is set to 128 MB. Each namespace object on the NameNode consumes approximately 150 bytes.

On DataNodes, data files are measured by disk space consumed—the actual data length—and not necessarily the full block size. For example, a file that is 192 MB consumes 192 MB of disk space and not some integral multiple of the block size. Using the default block size of 128 MB, a file of 192 MB is split into two block files, one 128 MB file and one 64 MB file. On the NameNode, namespace objects are measured by the number of files and blocks. The same 192 MB file is represented by three namespace objects (1 file inode + 2 blocks) and consumes approximately 450 bytes of memory.

Large files split into fewer blocks generally consume less memory than small files that generate many blocks. One data file of 128 MB is represented by two namespace objects on the NameNode (1 file inode + 1 block) and consumes approximately 300 bytes of memory. By contrast, 128 files of 1 MB each are represented by 256 namespace objects (128 file inodes + 128 blocks) and consume approximately 38,400 bytes. The optimal split size, then, is some integral multiple of the block size, for memory management as well as data locality optimization.

By default, Cloudera Manager allocates a maximum heap space of 1 GB for every million blocks (but never less than 1 GB). How much memory you actually need depends on your workload, especially on the number of files, directories, and blocks generated in each namespace. If all of your files are split at the block size, you could allocate 1 GB for every million files. But given the historical average of 1.5 blocks per file (2 block objects), a more conservative estimate is 1 GB of memory for every million blocks.

Replication

The default block replication factor (dfs.replication) is three. Replication affects disk space but not memory consumption. Replication changes the amount of storage required for each block but not the number of blocks. If one block file on a DataNode, represented by one block on the NameNode, is replicated three times, the number of block files is tripled but not the number of blocks that represent them.

With replication off, one file of 192 MB consumes 192 MB of disk space and approximately 450 bytes of memory. If you have one million of these files, or 192 TB of data, you need 192 TB of disk space and, without considering the RPC workload, 450 MB of memory: (1 million inodes + 2 million blocks) * 150 bytes. With default replication on, you need 576 TB of disk space: (192 TB * 3) but the memory usage stay the same, 450 MB. When you account for bookkeeping and RPCs, and follow the recommendation of 1 GB of heap memory for every million blocks, a much safer estimate for this scenario is 2 GB of memory (with or without replication).

Examples

Example 1: Estimating NameNode Heap Memory Used

Alice, Bob, and Carl each have 1 GB (1024 MB) of data on disk, but sliced into differently sized files. Alice and Bob have files that are some integral of the block size and require the least memory. Carl does not and fills the heap with unnecessary namespace objects.

Alice: 1 x 1024 MB file
  • 1 file inode
  • 8 blocks (1024 MB / 128 MB)
Total = 9 objects * 150 bytes = 1,350 bytes of heap memory
Bob: 8 x 128 MB files
  • 8 file inodes
  • 8 blocks
Total = 16 objects * 150 bytes = 2,400 bytes of heap memory
Carl: 1,024 x 1 MB files
  • 1,024 file inodes
  • 1,024 blocks
Total = 2,048 objects * 150 bytes = 307,200 bytes of heap memory

Example 2: Estimating NameNode Heap Memory Needed

In this example, memory is estimated by considering the capacity of a cluster. Values are rounded. Both clusters physically store 4800 TB, or approximately 36 million block files (at the default block size). Replication determines how many namespace blocks represent these block files.

Cluster A: 200 hosts of 24 TB each = 4800 TB.
  • Blocksize=128 MB, Replication=1
  • Cluster capacity in MB: 200 * 24,000,000 MB = 4,800,000,000 MB (4800 TB)
  • Disk space needed per block: 128 MB per block * 1 = 128 MB storage per block
  • Cluster capacity in blocks: 4,800,000,000 MB / 128 MB = 36,000,000 blocks
At capacity, with the recommended allocation of 1 GB of memory per million blocks, Cluster A needs 36 GB of maximum heap space.
Cluster B: 200 hosts of 24 TB each = 4800 TB.
  • Blocksize=128 MB, Replication=3
  • Cluster capacity in MB: 200 * 24,000,000 MB = 4,800,000,000 MB (4800 TB)
  • Disk space needed per block: 128 MB per block * 3 = 384 MB storage per block
  • Cluster capacity in blocks: 4,800,000,000 MB / 384 MB = 12,000,000 blocks
At capacity, with the recommended allocation of 1 GB of memory per million blocks, Cluster B needs 12 GB of maximum heap space.

Both Cluster A and Cluster B store the same number of block files. In Cluster A, however, each block file is unique and represented by one block on the NameNode; in Cluster B, only one-third are unique and two-thirds are replicas.