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 such as an instance failure to recover committed data that has not been written to the data files. The redo log files are only used for recovery. Online Redo Log Groups - A set of … [Read more...] about How to add and drop online redo log members and groups in Oracle
Archives for March 2021
Process are stuck in D-state due to multipathd
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 the queue will fail all stored IOs. If the outage will last, the admin might wish to unplug the queues right-away. The following … [Read more...] about Process are stuck in D-state due to multipathd
How to Disable RSH Server in CentOS/RHEL
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 and disable the rsh service to not start after reboot: # chkconfig rsh off Above command eg chkconfig does not stop rsh but … [Read more...] about How to Disable RSH Server in CentOS/RHEL
How to disable NFS client caching in CentOS/RHEL
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 the file could be old one. NFS Clients caches various NFS data. RFC 1813 for NFSv3 specification says: Clients can perform caching in varied manner. and also says: The NFS version 3 … [Read more...] about How to disable NFS client caching in CentOS/RHEL
How to Hot-add and Remove Logical Memory in CentOS/RHEL 7
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 … [Read more...] about How to Hot-add and Remove Logical Memory in CentOS/RHEL 7
MySQL Cluster requirements to avoid Single Point of Failure
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 come online, there must be at least two management nodes There must be at least two API nodes If one of the data nodes goes offline, an arbitrator must be present Note: The above … [Read more...] about MySQL Cluster requirements to avoid Single Point of Failure
What is an Arbitrator in MySQL Cluster
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 situation is undesirable because each set of nodes tries to behave as though it is the entire cluster. When cluster nodes go down, there are two possibilities. If more than 50% of the remaining nodes can … [Read more...] about What is an Arbitrator in MySQL Cluster
Oracle RAC Interview Questions – Coherence and Split-Brain
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 process(es) are no longer operational or using the said resources. What does "co-operating" mean? Co-operating … [Read more...] about Oracle RAC Interview Questions – Coherence and Split-Brain
How to disable swap in Linux
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 slower than reading from RAM, accessing memory pages there will result in a delay. The cache in Linux is used to speed up reads from disc - content which has been read once from disc is kept in … [Read more...] about How to disable swap in Linux
How To Install MySQL RPM packages in a different location to allow multiple versions (versions < 5.6.10)
Question: How to install the MySQL RPM packages in a different location to allow multiple versions and/or avoid compromising your existing installation? Note that it is generally not recommended to install the RPM package of MySQL in a different location. Install the RPM package(s) You can use the --relocate option for the rpm command to specify the location where you want to install MySQL. Important also, is the --noscripts option, which makes sure the post-installation scripts are not … [Read more...] about How To Install MySQL RPM packages in a different location to allow multiple versions (versions < 5.6.10)