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Online Tutoriale Kernel Boot Parameters

Posted by ascultradio on September 3, 2009

Kernel Boot Parameters :


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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