Blog/NFSv4 Client Tracking Daemon

NFSv4 Client Tracking Daemon

The largest photomosaic ever created from Hubble Space Telescope observations of the Andromeda galaxy. The galaxy's angular size makes it six times larger than the Moon, and the 2.5 billion pixel image shows more than 200 million stars. Credit: NASA, ESA, B. Williams (University of Washington)

NFSv4 introduces stateful tracking of client-server interactions, unlike NFSv3, which is stateless.

The Linux kernel NFSv4 server originally stored client tracking data statefully on disk, using the directory specified by /proc/fs/nfsd/nfsv4recoverydir. However, this method was deprecated in favour of an external daemon: nfsdcld.

The nfsdcld daemon, included in the nfs-utils package, manages NFSv4 client tracking in user space. This improves reliability compared to the legacy in-kernel tracking method.

Kernel Configuration and Future Changes

The legacy in-kernel client tracking mechanism is controlled by the kernel configuration option CONFIG_NFSD_LEGACY_CLIENT_TRACKING. If this option is disabled in the kernel, then nfsdcld required for NFSv4 client tracking to function correctly.

It is expected that CONFIG_NFSD_LEGACY_CLIENT_TRACKING may be removed entirely in a future kernel version, making nfsdcld mandatory for all NFSv4 deployments.

OpenRC Init Script for nfsdcld

I am using Gentoo and Alpine Linux, both OpenRC-based systems. They include nfsdcld, but not an init script to start it.

The following OpenRC init script ensures that rpc.pipefs is started and that the nfsd.ko kernel module is loaded before attempting to start nfsdcld itself.

#!/sbin/openrc-run
# Distributed under the terms of the GNU General Public License v2
name="nfsdcld"
description="NFSv4 Client Tracking Daemon"
command=/usr/sbin/nfsdcld
depend() {
    before nfs
    need rpc.pipefs
}
start_pre() {
    checkpath -d -m 0700 -o root:root /var/lib/nfs/nfsdcld
    if ! grep -qs nfsd /proc/filesystems ; then
        modprobe -q nfsd
    fi
}

When nfsdcld is not running, the NFS server logs a warning in dmesg:

[   30.739668] NFSD: Unable to initialize client recovery tracking! (-110)
[   30.739678] NFSD: Is nfsdcld running? If not, enable CONFIG_NFSD_LEGACY_CLIENT_TRACKING.
[   30.739682] NFSD: starting 90-second grace period (net f0000000)

When nfsdcld is running properly, the kernel confirms that it is being used:

[   46.361531] NFSD: Using nfsdcld client tracking operations.
[   46.361535] NFSD: starting 90-second grace period (net f0000000)