Besides the RAM there is a so called Swap, which is a virtual memory, where RAM content (pages) could be swapped-in in case there is not enough RAM available anymore. This swap is located on the disc and since disc reads and writes are slower than reading from RAM, accessing memory pages there will result […]
Archives for October 2015
CentOS / RHEL : How to collect sosreport
The “sosreport” is a tool to collect troubleshooting data on RHEL/CentOS systems. It generates a compressed tarball of debugging information that gives an overview of the most important logs and configuration of a Linux system, to be sent to Redhat Support. Among other things, the sosreport includes information about the installed rpm versions, syslog, network […]
CentOS / RHEL 7 : How to Reset root password
Resetting root password in CentOS/RHEL 7 is slightly different than the older versions. The post describes the procedure to reset the lost root password. Reboot and edit grub2 Reboot the system and press any key to stop the auto-boot from the default kernel. Press “e” after selecting the kernel line. Append rd.break to kernel When […]
lsof : Most commonly used examples
The lsof command lists open files, sockets, and pipes. You can use lsof command to see what files are held open (such as libraries or log files) and what ports daemons listen to. You can search for open files using lsof command. lsof should be run as the superuser (root) to see all open files. […]
CentOS / RHEL : How to identify/match LUN presented from SAN with underlying OS disk
The post mentions few ways to exactly identify/match the LUN presented from SAN with underlying OS disk. Method 1 Execute command below command to obtain Vendor, Model and Port, Channel, SCSI-ID, LUN # cat /proc/scsi/scsi Host: scsi2 Channel: 00 Id: 00 Lun: 29 Vendor: EMC Model: SYMMETRIX Rev: 5874 Type: Direct-Access ANSI SCSI revision: 05 […]
Replacing a Failed Mirror Disk in a Software RAID Array (mdadm)
The post describes the steps to replace a mirror disk in a software RAID array. In this example, we have used /dev/sda1 as the known good partition, and /dev/sdb1 as the suspect or failing partition. Before proceeding, it is recommended to backup the original disk. Before removing raid disks, please make sure you run the […]
CentOS / RHEL 5 : How to Configure kdump
Kdump is an advanced crash dumping mechanism. When enabled, the system is booted from the context of another kernel. This second kernel reserves a small amount of memory, and its only purpose is to capture the core dump image in case the system crashes. Since being able to analyze the core dump helps significantly to […]