The Geek Diary

Archives for March 2021

How to add and drop online redo log members and groups in Oracle

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Mirroring online redo logs The Oracle database maintains online redo log files to minimize the loss of data in the database. The redo log files record all changes made to data in the database buffer cache with some exceptions; for example, in the case of direct writes. Redo log files are used in a situation […]

Process are stuck in D-state due to multipathd

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For multipaths with queueing enabled and with all paths down, the kernel will store submitted IOs in a queue and resubmit them upon path recovery. Process submitting IOs to these multipath are stuck in D-state (uninterruptible sleep). The ‘no_path_retry 4‘ multipath parameter will instruct the kernel to unplug the queue after 4 failed path-tests. Unplugging […]

How to Disable RSH Server in CentOS/RHEL

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Question: How to completely disable RSH Server, and if necessary, uninstall it? To disable rsh-server: 1. Verify if rsh-server package is installed: # rpm -qa | grep rsh-server 2. If above command will return rsh-server package follow below action plan: # vi /etc/xinetd.d/rlogin specify there: disable = yes 3. Remove rsh entry from /etc/securetty file […]

How to disable NFS client caching in CentOS/RHEL

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Occasionally the NFS Client does not read data from NFS exports synchronously, such as when it was written by another NFS Client. For example, after NFS Client A writes a file by “echo hello > /NFS_mountpoint/testfile”, NFS Client B can not read the file with “No such file”, or can read but the contents of […]

How to Hot-add and Remove Logical Memory in CentOS/RHEL 7

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This post explains how to use memory hotplug to increase/decrease the amount of memory. To see (online/offline) state of memory section, read ‘state’ file. # grep line /sys/devices/system/memory/*/state /sys/devices/system/memory/memory0/state:online /sys/devices/system/memory/memory1/state:online /sys/devices/system/memory/memory2/state:online /sys/devices/system/memory/memory3/state:online /sys/devices/system/memory/memory4/state:online To set a memory[x] to online/offline: # echo online > /sys/devices/system/memory/memory[number]/state # echo offline > /sys/devices/system/memory/memory[number]/state To verify the current state: # […]

MySQL Cluster requirements to avoid Single Point of Failure

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In this post, we will discuss the minimum requirements to have no single point of failure for all services in a MySQL Cluster setup. To remove all single points of failure there are the following requirements to the Cluster configuration: There must be two copies of the data To allow API and data nodes to […]

What is an Arbitrator in MySQL Cluster

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If one or more nodes in a cluster fail, it is possible that not all cluster nodes will not be able to “see” one another. In fact, it is possible that two sets of nodes might become isolated from one another in a network partitioning, also known as a “split brain” scenario. This type of […]

Oracle RAC Interview Questions – Coherence and Split-Brain

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What is Split-Brain? The term “Split-Brain” is often used to describe the scenario when two or more co-operating processes in a distributed system, typically a high availability cluster, lose connectivity with one another but then continue to operate independently of each other, including acquiring logical or physical resources, under the incorrect assumption that the other […]

How to disable swap in Linux

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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. It could be a disk partition, Logical volume, or even a file. This swap is located on the disc and since disc reads and writes are […]