Rebooting the Magic Way
If you have ever had a hard drive fail on a remote server you may remember the feeling you had after trying to issue the following commands:
# reboot bash: /sbin/reboot: Input/output error # shutdown -r now bash: /sbin/shutdown: Input/output error
Obviously, there is a problem with your drive. These commands are failing because the kernel is unable to load the
/sbin/shutdown binaries from the disk so that it can execute them.
A fsck on the next boot might be able to correct whatever is wrong with the disk, but first you need to get the system to reboot. If your machine is located at a managed hosting provider then you could submit a reboot ticket, but you'll have to wait for someone to take responsibility.
Wouldn't it be nice if there was a way to ask the kernel to reboot without needing to access the failing drive? Well, there is a way, and it is remarkably simple.
The "magic SysRq key" provides a way to send commands directly to the kernel through the /proc filesystem. It is enabled via a kernel compile time option, CONFIG_MAGIC_SYSRQ, which seems to be standard on most distributions. First you must activate the magic SysRq option:
echo 1 > /proc/sys/kernel/sysrq
When you are ready to reboot the machine simply run the following:
echo b > /proc/sysrq-trigger
This does not attempt to unmount or sync filesystems, so it should only be used when absolutely necessary, but if your drive is already failing then that may not be a concern.
In addition to rebooting the system the sysrq trick can be used to dump memory information to the console, sync all filesystems, remount all filesystems in read-only mode, send SIGTERM or SIGKILL to all processes except init, or power off the machine entirely, among other things.
Also, instead of echoing into
/proc/sys/kernel/sysrq each time you can activate the magic SysRq key at system boot time using sysctl, where supported:
echo "kernel.sysrq = 1" >> /etc/sysctl.conf
If you would like to learn more about magic SysRq you can read the sysrq.txt file in the kernel documentation.
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
Join Linux Journal's Mike Diehl and Pat Cameron of Help Systems.
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With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide