The Problem The ntpd service is running, but executing ntpq -p fails as follows: # ntpq -p localhost.localdomain: timed out, nothing received ***Request timed out The Solution 1. Single "restrict" statement in /etc/ntp.conf This issue is most often seen when the default /etc/ntp.conf file is replaced with a custom one containing a single restrict default statement (with no other restrict lines). In that case, if the default restrict statement restricts querying (i.e., if "noquery" is … [Read more...] about ntpq fails with “timed out, nothing received” and “Request timed out” errors
How to Delete the Files Which are Older Than n Days WITHOUT Using find Command
Question: How to delete the files which are older than n days WITHOUT use find command? For some reason, the find command is not used to check which files are older than n days, as a substitute, 'tmpwatch' can be used The syntax is: # tmpwatch [time in hours] [directory] For example, to delete files under /root/testdir/ not accessed for last 45 days ( 24x45 = 1018 hours), run the following command: # tmpwatch 1080 /root/testdir/ You could also specify the atime/mtime/ctime etc, to … [Read more...] about How to Delete the Files Which are Older Than n Days WITHOUT Using find Command
How to determine file and free space fragmentation of OCFS2
Below is a list of steps to determine the extent fragmentation of disk space on OCFS2. Steps should work on an offlined o2image and provide an indication for fragmentation-related performance problems. Checking the fragments in OCFS2 volume 1.Use debugfs.ocfs2 to display on-disk OCFS2 filesystem structures on /dev/$your_ocfs2_volume, see the following example: # debugfs.ocfs2 /dev/sdb debugfs.ocfs2 1.6.3 debugfs: 2. Run "stat //global_bitmap": debugfs: stat //global_bitmap Inode: … [Read more...] about How to determine file and free space fragmentation of OCFS2
Connection using SSH to a Host Not in DNS/hosts Stalls for Some Time at Connection Initiation
ssh, scp or sftp to a node that doesn't appear in DNS or /etc/hosts file is slow to make an initial connection. After the connection is established, the speed is as expected. There are two cases to consider, see below. Note that in most environments this issue will not occur since IPs will be in /etc/hosts or DNS. The time is spent while resolving the IP address of the target/source hostname or finding the hostname for the target/source IP address and / or doing authentication based on the … [Read more...] about Connection using SSH to a Host Not in DNS/hosts Stalls for Some Time at Connection Initiation
How to check SCSI device’s storage path with “lsscsi” command
Question: How do I know a SCSI device's LUN ID (or storage path)? If you're using Red Hat Enterprise Linux 5, 6 or 7, you can install the lsscsi package and use the lsscsi command to view SCSI devices' storage path. # lsscsi [1:0:0:0] storage IET Controller 0001 - [1:0:0:1] disk IET VIRTUAL-DISK 0001 /dev/sdb In the outputs, "[1:0:0:1]" is in the format of [H:B:T:L]. H is the Host ID, B is the Bus ID, T is the Target ID, and L is the LUN ID. Below is … [Read more...] about How to check SCSI device’s storage path with “lsscsi” command
How to convert a collectl archive into a Performance Co-Pilot (PCP) archive
Question: I have captured a performance data archive using the collectl tool. How can I convert my collectl archive into a PCP archive so it can be analyzed using PCP monitoring tools? The following demonstrates converting a collectl performance data archive into a PCP archive, and then covers some very basic i/o analysis tasks using the PCP monitoring tools on the resulting archive. Note that the PCP archive used in these examples could also have been collected using PCP directly. How to … [Read more...] about How to convert a collectl archive into a Performance Co-Pilot (PCP) archive
DNS and DHCP Setup Example for Oracle Grid Infrastructure GNS
The Grid Naming Service (GNS) is a new feature in the 11.2 Grid Infrastructure (GI). Its design is to simplify the management of the network configuration of the Real Application Cluster (RAC). When GNS is used, it eliminates the manual allocation of the Node and Single Client Access Name (SCAN) Virtual Internet Protocol (VIP) addresses. It also eliminates the need to configure VIP names and SCAN names in a Domain Name Server (DNS) as GNS provides the name resolution for the cluster. What this … [Read more...] about DNS and DHCP Setup Example for Oracle Grid Infrastructure GNS
How to disable Xserver access in CentOS/RHEL
Question: How to check if Xserver access is enable or disabled? How to disable X server access if enabled? There are different ways to access or query local X server. XDMCP (query local X server to have separate remote session) Xorg based VNC (connect to local console from remote) Vino based vnc server (connect to local console from remote) Exporting DISPLAY variable to access local X server from remote. First 3 access mechanisms are not enabled by default. Follow the below steps … [Read more...] about How to disable Xserver access in CentOS/RHEL
Unmounting a Windows Share Fails in Linux
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, following error is seen; # umount: /mycloud: 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 Unmounting a Windows Share Fails in Linux
How to interpret Linux martian source messages
What is a Martian Packet? The IANA defines a Martian packet as one which arrives on an interface where the interface does not use that network. For Linux, it's any packet that arrives on an interface which is not configured for that subnet in any way. Any martian packet notice should be investigated. Martian packets: Are frequently used in hacking intrusion. May be a symptom of a misconfigured server elsewhere on the network. May indicate a network infrastructure issue. If … [Read more...] about How to interpret Linux martian source messages