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 6
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 Permanently set the ethtool settings in CentOS/RHEL 6
Modifications made with the ethtool command are by default disappeared after a reboot. For permanent settings, the interface configuration file has to be edited. For each network device, there will be a file in the /etc/sysconfig/network-scripts/ For example, for device eth0 there will be /etc/sysconfig/network-scripts/ifcfg-eth0 containing the properties of the device. A line can be added to the file to automatically implement the settings allowed by the "ethtool -s" command. For example: # … [Read more...] about How to Permanently set the ethtool settings in CentOS/RHEL 6
How to Create a MySQL Docker Container for Testing
Docker is a container runtime environment that allows programs to operate in a jailed environment without any required external dependencies. Containers are similar in some concepts to virtual machines, however, they do not use a hypervisor and run in a single kernel instance, often sharing the instance with other containers. One of the prime features of containerization is the lack of external dependencies; the container has all of the runtime libraries and components needed to run the … [Read more...] about How to Create a MySQL Docker Container for Testing
How to enable additional scsi logging in CentOS/RHEL
Question: How to turn on scsi extended debug messages? What are the field/flag definitions of scsi_logging_level? Additional scsi logging messages can be enabled by writing to /proc/sys/dev/scsi/logging_level either via use of the echo or preferably the sysctl command. The kernel parameter consists of ten packed fields, each 3 bits in length. Each field can have a value of 0 to 7. The higher the field's value, the more verbose the logging of messages associated with that field type. NOTE: … [Read more...] about How to enable additional scsi logging in CentOS/RHEL
When to use rescan-scsi-bus.sh -i (LIP flag) in CentOS/RHEL
The "rescan-scsi-bus.sh" can be used to scan new LUNs in a CentOS/RHEL machine. When 'rescan-scsi-bus.sh -i' is run, script execute as well a LIP_RESET (ISSUE_LIP) which may cause a disruption in I/O on the server and even cause an outage in case of a system running on heavy load. What is LIP (Loop Initialization Protocol)? LIP scans the interconnect and causes the SCSI layer to be updated to reflect the devices currently on the bus. A LIP is, essentially, a bus reset, and will cause device … [Read more...] about When to use rescan-scsi-bus.sh -i (LIP flag) in CentOS/RHEL
How to Disable rpc.quotad Service in CentOS/RHEL 6 and 7
This post explains how to disable rpc.quotad service on CentOS/RHEL 6 or 7 machines. The protocol used by rpc.rquotad is an unsecured and obsolete protocol and should be disabled. Note: This procedure can disrupt NFS clients relying on disk quotas. To avoid issues, schedule a maintenance outage and stop these NFS clients. For CentOS/RHEL 6 or earlier 1. Edit the configuration file /etc/sysconfig/nfs to have an entry which reads RQUOTAD="no". For example: # grep RQUOTAD … [Read more...] about How to Disable rpc.quotad Service in CentOS/RHEL 6 and 7
CentOS/RHEL : Unmounting a Windows Share Fails – “device is busy”
The Problem Unable to unmount a Windows share mount point using umount command. This Windows share is not being used by any processes recently. When executing umount command, the following error is seen; umount: /data01: device is busy. (In some cases useful info about processes that use the device is found by lsof(8) or fuser(1)) The Solution The error 'umount: device is busy' means that the mount point is being held up by the server due to some reason. An lsof command is executed … [Read more...] about CentOS/RHEL : Unmounting a Windows Share Fails – “device is busy”
How to activate a “SUSPENDED” LVM physical volume
When suspending a device using the command dmsetup suspend, any I/O that has already been mapped by the device but has not yet completed will be flushed. For example, lvextend command was executed and for some reason, it did not finish. This will keep the volume in suspended mode and all I/O will be blocked. Most of the time, this indicates a problem with the underlying physical volumes and should be investigated further. How to confirm if PV is SUSPENDED Check for the # dmsetup info … [Read more...] about How to activate a “SUSPENDED” LVM physical volume
Downgrading an rpm package to a lower version (using “rpm” command)
Question: How to downgrade an rpm package in CentOS/RHEL 6 system to a lower version using the "rpm" command? Its not really recommended to use rpm command instead of yum to downgrade an rpm package. But in case if you don't have a choice use the below procedure. Pre-requisites Make sure you have downloaded all the packages and its dependencies for the lower version. Downgrading rpm version For the purpose of this post we are downgrading the systemd packages from … [Read more...] about Downgrading an rpm package to a lower version (using “rpm” command)