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Chapter 4.
Kernel Boot Parameters
4.1. General
The Linux kernel accepts boot parameters when the kernel is started. Very often it is used to provide
information to the kernel about hardware parameters where the kernel would have problems or to
overwrite default values. Red Hat Enterprise Linux 3 uses 2.4 based kernels, Red Hat Enterprise Linux
4 and 5 use 2.6 based kernels.
For a list of kernel parameters in Red Hat Enterprise Linux 4 and 5, see /usr/share/doc/
kernel-doc-2.6.9/Documentation/kernel-parameters.txt. This file does not exist if
the kernel-doc RPM is not installed. And for a list of kernel parameters in Red Hat Enterprise
Linux3 and Red Hat Enterprise Linux 2.1, see /usr/src/linux-2.4/Documentation/kernel-
parameters.txt which comes with the kernel-doc RPM.
4.2. The I/O Scheduler
Starting with the 2.6 kernel, for example Red Hat Enterprise Linux 4 or 5, the I/O scheduler can be
changed at boot time which controls the way the kernel commits reads and writes to disks. For more
information on various I/O scheduler, see
1
and the 2.6 Kernel
. Red Hat Enterprise Linux 5 allows users to change I/O schedulers dynamically,
one way this can be done is by executing the command echo sched_name > /sys/block/
<sdx>/queue/scheduler.
The Completely Fair Queuing (CFQ) scheduler is the default algorithm in Red Hat Enterprise Linux
4 which is suitable for a wide variety of applications and provides a good compromise between
throughput and latency. In comparison to the CFQ algorithm, the Deadline scheduler caps maximum
latency per request and maintains a good disk throughput which is best for disk-intensive database
applications. Hence, the Deadline scheduler is recommended for database systems. Also, at the time
of this writing there is a bug in the CFQ scheduler which affects heavy I/O, see Metalink Bug:5041764.
Even though this bug report talks about OCFS2 testing, this bug can also happen during heavy IO
access to raw or block devices and as a consequence could evict RAC nodes.
To switch to the Deadline scheduler, the boot parameter elevator=deadline must be passed to the
kernel that is being used. Edit the /etc/grub.conf file and add the following parameter to the kernel
that is being used, in this example 2.4.21-32.0.1.ELhugemem:
title Red Hat Enterprise Linux Server (2.6.18-8.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-8.el5 ro root=/dev/sda2 elevator=deadline
initrd /initrd-2.6.18-8.el5.img
This entry tells the 2.6.18-8.el5 kernel to use the Deadline scheduler. Make sure to reboot the
system to activate the new scheduler.
1
http://www.redhat.com/magazine/008jun05/features/schedulers/
Choosing an I/O Scheduler for Red Hat Enterprise Linux 4
9
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