Memory and Disk Space

This section outlines required and recommended memory, storage, and disk capacities.

Minimum Memory

A minimum of 16 GB total memory is required on a node. MapR recommends at least 16 GB for a production environment, and typical MapR production nodes have 32 GB or more.

Run free -g to display total and available memory in gigabytes.

$ free -g
              total        used        free      shared      buffers      cached
Mem:              3           2           1           0            0           1
-/+ buffers/cache:            0           2
Swap:             2           0           2

If the free command is not found, there are many alternatives: grep MemTotal: /proc/meminfo, vmstat -s -SM, top, or various GUI system information tools.

Using the numad service is not recommended, since it has not been tested and validated with MapR software. Using numad can cause artificial memory constraints to be set, which can lead to performance degradation under load. To disable numad:

  1. Stop the service by issuing the command service numad stop.
  2. Set the numad service not to start on reboot: chkconfig numad off

Using overcommit is not recommended because it can lead to the kernel memory manager stopping processes to free memory, resulting in stopped MapR processes and system instability. Set vm.overcommit_memory to 0:

  1. Edit the file /etc/sysctl.conf and add the following line: vm.overcommit_memory=0
  2. Save the file and run: sysctl -p
Note: You can try MapR on non-production equipment, but under the demands of a production environment, memory needs to be balanced against disks, network, and CPU.


MapR software manages raw, unformatted devices directly to optimize performance and offer high availability. For data nodes, allocate at least 3 unmounted physical drives or partitions for MapR storage. MapR software uses disk spindles in parallel for faster read/write bandwidth and therefore groups disks into sets of three.

Minimum Disk Allocation: MapR software requires a minimum of one disk or partition for MapR data. However, file contention for a shared disk decreases performance. In a typical production environment, multiple physical disks on each node are dedicated to the distributed file system, which results in much better performance.

Drive Configuration

Do not use RAID or Logical Volume Management with disks that will be added to a MapR node. While MapR software supports these technologies, using them incurs additional setup overhead and can affect your cluster's performance. Due to the possible formatting requirements that are associated with changes to the drive settings, configure the drive settings prior to installing MapR.

If you have a RAID controller, configure it to run in HBA mode. For LSI MegaRAID controllers that do not support HBA, configure the following drive-group settings for optimal performance:

Property (The actual name depends on the version) Recommended Setting
Stripe Size >=256K
Cache Policy or I/O Policy Cached IO or Cached
Read Policy Always Read Ahead or Read Ahead
Write Policy Write-Through
Disk Cache Policy or Drive Cache Disabled
Note: Enabling the Disk Cache policy can improve performance. However, enabling the Disk Cache policy is not recommended because it increases the risk of data loss if the node loses power before the disk cache is committed to disk.

Minimum Disk Space

OS Partition. Provide at least 10 GB of free disk space on the operating system partition.

MapR-FS. Provide the higher of 8 GB of free disk space or the memory allocated to MapR file system. Note that the disk space should be greater than the memory allocated to MapR file system.

Disk. Provide 10 GB of free disk space in the /tmp directory and 128 GB of free disk space in the /opt directory. Services, such as ResourceManager and NodeManager, use the /tmp directory. Files, such as logs and cores, use the /opt directory.

Swap space. For production systems, provide at least 4 GB of swap space. If you believe more swap space is needed, consult the swap-space recommendation of your OS vendor. The amount of swap space that a production system needs can vary greatly depending on the application, workload, and amount of RAM in the system.  Note that the MapR Installer generates a warning if your swap space is less than 10% of main memory or less than 2 GB.

ZooKeeper. On ZooKeeper nodes, dedicate a partition, if practicable, for the /opt/mapr/zkdata directory to avoid other processes filling that partition with writes and to reduce the possibility of errors due to a full /opt/mapr/zkdata directory. This directory is used to store snapshots that are up to 64 MB. Since the four most recent snapshots are retained, reserve at least 500 MB for this partition. Do not share the physical disk where /opt/mapr/zkdata resides with any MapR File System data partitions to avoid I/O conflicts that might lead to ZooKeeper service failures.

Virtual Memory (swappiness)

Swappiness is a setting that controls how often the kernel copies the contents of RAM to swap. By setting vm.swappiness to the right value, you can prevent the system from swapping processes too frequently, but still allow for emergency swapping (instead of killing processes). For all Linux distributions, the MapR recommendation is to set vm.swappiness to 1.

To check the current value for vm.swappiness:
cat /proc/sys/vm/swappiness
To change the value:
sudo sysctl vm.swappiness=1
Note: The value of vm.swappiness can revert to a system default setting if you reboot the node. After rebooting, you might need to check and reset the value.