FedFsNfsServer0.8

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Revision as of 17:56, 24 October 2012

Project: fedfs-utils

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Introduction

This article describes how to configure a Linux NFS server to resolve junctions. Once a file server can resolve junctions in its local file systems, it can participate in a FedFS domain.

A junction is a link between two shared file systems. The two shared file systems may not reside on the same server. There may be more than one replica of the linked-to shared file system, stored on several different file servers. Each replica of the linked-to file system is called a location.

Junctions store a list of locations. Each location in the list is represented as an export path paired with a server name or IP address. When a file-access client encounters a junction, the file server (via a referral) provides the stored list of locations to the client, who chooses a location and mounts it.

A file server must convert the information stored in a junction into a list of locations that's in a format which is correct for the protocol clients are using to access the server (say, NFSv4). The server-side process of recognizing a junction and converting its contents is known as junction resolution.

A single junction is useful for linking together two shared file systems. A web of junctions can be used to create a namespace that crosses between multiple file systems on multiple file servers. FedFS uses junctions for just such a purpose.

On Linux, a junction is denoted by a directory with special mode bits and an extended attribute that contains the junction location list. The NFS server must convert a junction's location information into an NFSv4 referral which can be sent to NFS clients.

When the Linux in-kernel NFS server encounters a junction, it resolves the junction by performing an upcall to mountd. It is mountd that actually reads a junction and converts it to information that the kernel NFS server can put on the wire as an NFSv4 referral.

Installing or Building fedfs-utils

On Fedora 18, two packages are needed to enable junction resolution on NFS servers:

# yum install fedfs-utils-server fedfs-utils-lib

If your distribution does not provide a fedfs-utils package, you need to build fedfs-utils-0.8 yourself:

• Installing or Building fedfs-utils-0.8

Additional info

Instead of installing the whole package, you may choose to install just the server components. The plug-in library:

# cd fedfs-utils-0.8/src/plug-ins ; make install

The header describing the library's API is needed to enable junction support while building nfs-utils:

# cd fedfs-utils-0.8/src/include ; make install

And finally, the "nfsref" command:

# cd fedfs-utils-0.8/src/nfsref ; make install

Build nfs-utils

Now download and unpack the latest nfs-utils tarball on the same system where you built fedfs-utils. Build nfs-utils, and replace rpc.mountd on your NFS server. The configure options are complicated; I usually copy them from the latest nfs-utils RPM spec file from my distribution.

$ ./configure yada yada
$ make
$ sudo install utils/mountd/mountd /usr/sbin/rpc.mountd

Then restart the NFS service.

# systemctl restart nfs-server.service