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Mounting HDFS via FUSE and Exporting Over NFS

By Anonymous
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This guide shows how to mount an HDFS filesystem using FUSE and then export part of it over NFS so that remote systems can access HDFS like a local filesystem. The approach is based on classic Hadoop and Linux tools, and includes notes on security, user mapping, and kernel limitations.

In some environments, it can be useful to make an HDFS filesystem available across networks as an exported share. This walkthrough describes a working scenario using Linux and Hadoop with tools that are typically included in older Hadoop distributions.

The setup uses hadoop-fuse-dfs and libhdfs to mount HDFS locally, and then exports that mount over NFS. Replace namenode.local and <PORT> with values appropriate for your cluster.

1. Install FUSE and libhdfs

yum install hadoop-0.20-fuse.x86_64 hadoop-0.20-libhdfs.x86_64

2. Create a mountpoint

mkdir /hdfs-mount

3. Test mounting HDFS via FUSE

hadoop-fuse-dfs dfs://namenode.local:<PORT> /hdfs-mount -d

If the mount succeeds, you should see output similar to: 

INFO fuse_options.c:162 Adding FUSE arg /hdfs-mount
INFO fuse_options.c:110 Ignoring option -d
unique: 1, opcode: INIT (26), nodeid: 0, insize: 56
INIT: 7.10
flags=0x0000000b
max_readahead=0x00020000
INFO fuse_init.c:101 Mounting namenode.local:<PORT>
INIT: 7.8
flags=0x00000001
max_readahead=0x00020000
max_write=0x00020000
unique: 1, error: 0 (Success), outsize: 40

Once you see Success, you can stop the foreground process with Ctrl+C. The command above is mainly for testing.

4. Configure the mount at boot time

To mount HDFS automatically at boot, add an entry to /etc/fstab: 

echo "hadoop-fuse-dfs#dfs://namenode.local:<PORT> /hdfs-mount fuse usetrash,rw 0 0" >> /etc/fstab

Then test the configuration:

# mount -a
# mount
[...]
sunrpc on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw)
fuse on /hdfs-mount type fuse (rw,nosuid,nodev,allow_other,default_permissions)

If you see the FUSE mount entry for /hdfs-mount, the configuration is working.

5. Tuning JVM memory for FUSE

Each FUSE process uses a JVM. To tune memory settings, inspect and adjust: 

/etc/default/hadoop-0.20-fuse

Here you can configure Java heap size and other runtime parameters to fit your workload and hardware limits.

6. Export HDFS via NFS (unsecure)

The next step is to export part of the FUSE-mounted HDFS via NFS. Note that this is considered unsecure and should be used only in trusted environments. User IDs and permissions are mapped at the OS level.

6.1 Select the user for NFS exports

Assume you want to export data using the hdfs user. Check its UID and GID: 

# id hdfs
uid=104(hdfs) gid=105(hdfs) groups=105(hdfs),104(hadoop) context=root:staff_r:staff_t:SystemLow-SystemHigh

6.2 Create the NFS exports configuration

Define an export for the HDFS user directory in /etc/exports: 

# cat /etc/exports
/hdfs-mount/user   (fsid=111,rw,wdelay,anonuid=104,anongid=105,sync,insecure,no_subtree_check,no_root_squash)

Explanation (simplified):

  • rw: read-write
  • fsid=111: unique filesystem ID (see man 5 exports)
  • wdelay: write delay
  • anonuid=104, anongid=105: map anonymous users to the hdfs user and group
  • sync: synchronous writes
  • insecure: allow connections from non-privileged ports
  • no_subtree_check, no_root_squash: disable subtree checks and root squashing

Exporting only the /hdfs-mount/user directory helps protect system-related paths such as /mapred or other service directories from accidental modification.

Restart the NFS server to apply the configuration:

# service nfs restart

7. Using HDFS as a “local” filesystem via NFS

From a client machine, you can now mount the exported NFS share and work with HDFS as if it were a local directory: 

# mount -t nfs <NFS_SERVER_HOST>:/hdfs-mount/user /mnt/hdfs-user

After mounting, you can create or copy job definitions and files directly into HDFS through this NFS path. Keep in mind:

  • All file operations are translated through FUSE and libhdfs into HDFS calls.
  • Permissions and ownership are mapped to the local hdfs user as configured.
  • Using root on clients is a bad idea; stick to regular users and rely on the mapping.

8. Security and compatibility notes

This pattern relies on classic kernels and older Hadoop components:

  • It only works reliably from Linux kernel 2.6.27 upwards (as originally tested).
  • NFS exports based on FUSE-mounted HDFS are not recommended for multi-tenant or untrusted environments.
  • For modern clusters, consider HDFS NFS Gateway, WebHDFS, or object-store abstractions instead.

Within its constraints, this approach can still be useful for legacy clusters and simple, controlled use cases where administrators need quick, filesystem-style access to HDFS.

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