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Online Tutoriale Kernel Boot Parameters

Posted by ascultradio on September 3, 2009

Kernel Boot Parameters :

General

The Linux kernel accepts boot parameters when the kernel is started. Very often it’s used to provide information to the kernel about hardware parameters where the kernel would have issues/problems or to overwrite default values.

For a list of kernel parameters in RHEL4, 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 RHEL3 and RHEL2.1, see /usr/src/linux-2.4/Documentation/kernel-parameters.txt which comes with the kernel-doc RPM.

I/O Scheduler

Starting with the 2.6 kernel, i.e. RHEL 4, 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 Choosing an I/O Scheduler for Red Hat Enterprise Linux 4 and the 2.6 Kernel.

The Completely Fair Queuing (CFQ) scheduler is the default algorithm in RHEL4 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/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’s being used. Edit the /etc/grub.conf file and add the following parameter to the kernel that’s 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.

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