Databases Archives - Page 85 of 89

What is apache Cassandra?

30/08/201914/04/2016 by Philip

Cassandra is a fast distributed database.
It has several defining features:

Built in high availability. – Any node can handle read and write requests and your data is replicated to x nodes so regardless of which node (or even a data center) goes down, you will still have access to read and write your data.
Linear Scalability. – Doubling the number of (identical) nodes should double the write performance. Its basically as simple as that was all nodes can handle all operations and there is no central control.
Predictable performance. (i.e. doubling the number of identical nodes should double the write throughput)
no single point of failure. -nodes can go down and come back up without the front end application becoming aware of it.
Multiple Data Centres catered for and taken advantage of out as standard.
Built to run on commodity hardware – so you can run it on lots of $1000 servers rather than 1 or 2 $100000 servers.
Easy to manage operationally. – The system is designed to need very little ops input.

Relational Databases and Big Data workloads.

13/04/2016 by Philip

This intro to Cassandra is taken from the DataStax course. I don’t necessarily agree with everything – particularly their take on what a traditional RDBMS can and can’t do but I have included their view here for completeness.

Cassandra is designed for ‘Big Data’ workloads. Im order to understand the characteristics of Big Data, lets first define ‘Small Data’:

This would typically be a volume of storage that would fit on 1 machine and a RDBMS is typically fine and able to handle the number of operations and the quantity of data. The system will support a number of concurrent users in the hundreds. It fully supports ACID.

When you want to scale such a system, you are going to do it vertically first – with a bigger host, more RAM or processors.

Can Relational databases support big data?

Get all of the tables linked by a Foreign Key

20/01/2016 by Philip

Here is a handy little PostgreSQL query that will list all of the tables linked by a foreign key to the table YOUR_TABLE. The output will include the keys on each side. This can be very useful if you want to build up a map of your database through the relationships between the tables. select …

How to kill all sessions in Postgres so that they stay dead.

20/01/201618/01/2016 by Philip

Have you ever tried to drop or rename a postgres database or even create another database from one as a template, only to be confronted by the dreaded message: postgres=# create database test with template a_database; ERROR: source database “a_database” is being accessed by other users DETAIL: There are 40 other sessions using the database. …

What is an Oracle Service

10/02/2014 by Philip

Oracle services were a feature introduced in Oracle 10g. Their function is to simplify workload management by allowing you to group applications that share traits such as thresholds, priorities and attributes. The Oracle database is presented as a service and so you always have at least one service running. It is good practice to create …

Oracle ASM Mirroring and Failure groups.

09/02/2014 by Philip

Oracle ASM keeps redundancy quite simple by providing only 3 options. you can choose to have no redundancy or mirroring provided by the ASM instance. This is the ‘External’ option and assumes that you are handling mirroring somewhere else perhaps through hardware raid on your storage network. You can chose ‘normal’ 2 way mirroring r …

How to modify Voting disks in ASM

09/02/2014 by Philip

The process for modifying a votedisk that is stored on ASM differs from the process when the votedisk is stored on some other medium. When your voting disks are stored on ASM you can move them off to non ASM storage with the replace command: $ crsctl replace votedisk [path_to_new_votedisk] (this can be done from …

How ASM Dynamic Volume Manager presents its Volumes.

08/02/2014 by Philip

An ASM instance forms and presents is the underlying structure for an ASM dynamic volume. ASM provides a disk group which is really a logical container for physical space. ASM is very efficient at storing data files but there are many types of files that cannot be stored on ASM and it cannot be read …

How is the SGA in an ASM instance made up

08/02/2014 by Philip

An ASM instance has its own SGA but it is not the same as the SGA in a database instance. The 4 main sections of the SGA in an ASM instance are: ASM Cache – this is used during rebalancing operations for reding and writing blocks. Large Pool – Used for Parallel Operations. Shared Pool …

How to create more copies of Voting Disks in Oracle 11g RAC

05/02/2014 by Philip

You cannot simply add more copies of voting disks to increase redundancy because the number of voting disks is managed by Oracle ASM and determined by the redundancy of the disk group. To increase the number of voting disks, you need to move them to a disk group with higher redundancy. So, create the new …