Archive for July, 2009

Managing loop devices on CentOS and Fedora Linux hosts

Linux loop devices provide a way to mount ordinary files as block devices. This capability allows you to easily access an ISO image that resides on disk, mount and unmount encrypted devices (the dm-crypt and fuse encryption module may be a better solution for this), or test out new file systems using plain old files. […]

Increasing the number of available file descriptors on Centos and Fedora Linux Servers

While debugging an application a few weeks back, I noticed the following error in the application log: Cannot open file : Too many open files The application runs as an unprivileged user, and upon closer inspection I noticed that the maximum number of file descriptors available to the process was 1024: $ ulimit -n 1024 […]

Managing /etc/sysctl.conf with the sysctl utility

The Linux kernel provides the sysctl interface to modify values that reside under the /proc/sys directory. Sysctl values are typically stored in /etc/sysctl.conf, and are applied using the sysctl utility. To set a sysctl variable to a specific value, you can run sysctl with the “-w” (change a specific sysctl variable) option: $ sysctl -w […]

Enabling IPv4 forwarding on Centos and Fedora Linux servers

When I was playing around with KeepAlived, I managed to create a few HA scenarios that mirrored actual production uses. One scenario was creating a highly available router, which would forward IPv4 traffic between interfaces. To configure a CentOS or Fedora Linux host to forward IPv4 traffic, you can set the “net.ipv4.ip_forward” sysctl to 1 […]

Global search feature added to prefetch.net

I have posted a lot of material to my website over the past few years, and needed a way to easily locate content. To make finding material super easy, I recently added a global search page. I’m not sure if folks will find this useful, but I thought I would throw it out there in […]

Measuring TCP and UDP throughput between Linux and Solaris hosts

I have been assisting a friend with tuning his Netbackup installation. While debugging the source of his issues, I noticed that several jobs were reporting low throughput numbers. In each case the client was backing up a number of large files, which should have been streamed at gigabit Ethernet speeds. To see how much bandwidth […]

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