Sometimes not all log messages in /var/log/dmesg can be captured after Linux OS/kernel boots up because of too many block devices attached to the server. This post illustrates another way to record all bootup log messages besides configuring the serial console. Basically kernel parameter "log_buf_len" can be configured to increase kernel log buffer size, Name: log_buf_len, Set the size of the kernel log buffer. Synopsis: log_buf_len= n [ KMG ] Description: Set the size of the … [Read more...] about How to Capture More Logs in /var/log/dmesg for CentOS/RHEL
CentOS/RHEL
Unable to Start RDMA Services on CentOS/RHEL 7
The Problem The RDMA service fails to start on CentOS/RHEL 7 machine. 'journalctl -r' command shows the below errors: Oct 11 11:01:15 geeklab systemd-modules-load: Failed to insert 'rds_rdma': Address family not supported by protocol Oct 11 11:01:15 geeklab systemd-modules-load: Failed to insert 'rds_rdma': Address family not supported by protocol Oct 11 11:01:15 geeklab systemd-modules-load: Failed to insert 'resilient_rdmaip': Address family not supported by protocol Oct 11 11:01:15 … [Read more...] about Unable to Start RDMA Services on CentOS/RHEL 7
How to rename a KVM VM with virsh
The syntax is to rename a KVM VM with virsh is as follows: # virsh domrename {domain} {new-name} Let's see an example of renaming a VM using virsh. 1. To rename VM from serverA to serverB, first shutdown the VM: # virsh shutdown serverA 2. Now to rename a VM, run: # virsh domrename serverA serverB Domain successfully renamed 3. To start the VM/domain: # virsh start serverB Rename KVM domain with virsh and XML 1. First get domain information in XML and save it to a new … [Read more...] about How to rename a KVM VM with virsh
How to set “max_report_luns” and “max_luns” on CentOS/RHEL 7 to scan more than 16,383 LUNs
From CentOS/RHEL 7, the default value of parameter max_report_luns has been increased from 512 to 16383. Consequently, CentOS/RHEL 7.x can now discover more than 511 LUNs when the system scans the SCSI interconnect using the Report LUNs mechanism. Increasing this value beyond 16383 is generally not required. But if you want to increase it, then change the values for either max_report_luns or max_luns in the kernel options line in /etc/default/grub. Note: The specific kernel option line within … [Read more...] about How to set “max_report_luns” and “max_luns” on CentOS/RHEL 7 to scan more than 16,383 LUNs
How to set “max_report_luns” and “max_luns” on CentOS/RHEL 6 to scan more than 512 LUNs
The default limit of maximum luns that can be presented in a CentOS/RHEL 6 is 512. But in many cases, you may want to increase this limit. There is a parameter of the SCSI core module to control the number of multiple LUNs. View Current Settings To view the current setting of "max_luns" parameter, use the below command: # systool -v -m scsi_mod Module = "scsi_mod" Attributes: uevent = Parameters: default_dev_flags = "0" eh_deadline = "-1" … [Read more...] about How to set “max_report_luns” and “max_luns” on CentOS/RHEL 6 to scan more than 512 LUNs
How to Check the Size og yum Channels in CentOS/RHEL
Question: How to find out how large a yum channel repository is, to know how much data the channel will sync to a local yum repository? To see the actual size of the repository use either the 'yum repolist -v' or 'yum repoinfo' command. Example for checking the size of "CentOS-8 - AppStream" yum channel Run the command as shown below to get the size of the Repo "CentOS-8 - AppStream". The command works even without the double quotes around the repo name. # yum repoinfo "CentOS-8 - … [Read more...] about How to Check the Size og yum Channels in CentOS/RHEL
Repairing filesystem issues at Boot in CentOS/RHEL 7 and 8
This post focuses on manually repairing the file-system configuration or corruption issues that stop the boot process. Diagnosing and fixing file system issues Errors in /etc/fstab and corrupt file systems can stop a system from booting. In most cases, systemd drops to an emergency repair shell that requires the root password. The following table lists some common errors and their results. Common File System Issues PROBLEM RESULT Corrupt file system systemd attempts to repair the … [Read more...] about Repairing filesystem issues at Boot in CentOS/RHEL 7 and 8
How to Use Udev Rules to Create oracleasm Disks in CentOS/RHEL 8
This post will help to configure Oracleasm disks using Udev rules in 2 node cluster. On first node 1. List the disks: [root@node01 ~]# lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 2G 0 disk sdb 8:16 0 2G 0 disk Note: Create partitions for the disks. 2. Find the disk ID: [root@node01 ~]# udevadm info --query=all --name=/dev/sda | egrep "WWN|SERIAL" E: ID_SCSI_SERIAL=[id] E: ID_SERIAL=0QEMU_QEMU_HARDDISK_AAAAAAAAA E: … [Read more...] about How to Use Udev Rules to Create oracleasm Disks in CentOS/RHEL 8
How to Configure an NFSv4-only Client using nfsconf in CentOS/RHEL 8
The nfsconf tool CentOS/RHEL 8 introduces the nfsconf tool to manage the NFS client and server configuration files under NFSv4 and NFSv3. Configure the nfsconf tool using /etc/nfs.conf (the /etc/sysconfig/nfs file from earlier versions of the operating system is deprecated now). Use the nfsconf tool to get, set, or unset NFS configuration parameters. The /etc/nfs.conf configuration file is composed of multiple sections starting with a key word in square brackets ([keyword]) with value … [Read more...] about How to Configure an NFSv4-only Client using nfsconf in CentOS/RHEL 8
Beginners Guide to Tuning Profiles in CentOS/RHEL
Tuning Systems System administrators can optimize the performance of a system by adjusting various device settings based on a variety of use case workloads. The tuned daemon applies tuning adjustments both statically and dynamically, using tuning profiles that reflect particular workload requirements. Configuring Static Tuning The tuned daemon applies system settings when the service starts or upon the selection of a new tuning profile. Static tuning configures predefined kernel parameters … [Read more...] about Beginners Guide to Tuning Profiles in CentOS/RHEL