# Examples of estimating NameNode heap memory

You can estimate the heap memory by consider either the number of file inodes and blocks, or the cluster capacity.

## 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.