Tuning NFS client read ahead on SLE 10 and 11

This document (7003356) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 11

SUSE Linux Enterprise Server 10 Service Pack 2

SUSE Linux Enterprise Desktop 11

SUSE Linux Enterprise Desktop 10 Service Pack 2

Situation

How is NFS client read-ahead tuned on SLE 10 and 11?

Resolution

SLES 11:

In SLES 11, the tunable is present at:

/sys/class/bdi/X:Y/read_ahead_kb

Note that this is tunable *after* the file system is mounted. "X:Y" is variable, although with NFS it will typically be 0:something. Normally "backing devices" are block device. But for NFS, a synthetic "backing device" is created to store these parameters. In that case the 'X' is 0, and the 'Y' is some random number.

If a particular NFS mount used "0:18", then the place to tune the read ahead for that mount would be:

/sys/class/bdi/0:18/read_ahead_kb

The system can tell you what the 0:n number through:

stat -c '%d' /mount/point

In theory, a script could be used to inspect and set the read_ahead_kb for a given file system. Here's a possible example, called set-ra.sh:

#!/bin/sh

# usage: To display current value: set-ra.sh </mount/point>

# To set a new value: set-ra.sh </mount/point> <new_value>

case $# in

1) cat /sys/class/bdi/0:`stat -c '%d' "$1"`/read_ahead_kb

;;

2) echo $2 > /sys/class/bdi/0:`stat -c '%d' "$1"`/read_ahead_kb

;;

esac

#----------end-------------

This script is just an example, it is not highly tested.

SLES 10 SP2:

On SLES 10 SP2, the live value is stored at /proc/sys/fs/nfs/nfs_max_readahead

For dynamic tuning, set it with (i.e. if 32 is the desired value):

echo 32 > /proc/sys/fs/nfs/nfs_max_readahead

This tunable only takes effect if it is in place before the mount is done. So to put this into effect dynamically, you'd need something in the order of:

umount

tune

mount

This could also be set at boot, before /etc/fstab mounts are done, by editing /etc/sysctl.conf and adding:

fs.nfs.nfs_max_readahead = 20

However, if the system is not up-to-date, this might not come into effect during boot, due to timing issues between sysctl and nfs initialization. Without current updates (May 2009), the following may be needed after boot:

sysctl -p

rcnfs restart

Disclaimer

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