linux

fritshoogland's picture

The curious case of the missing semctl call

This article is about the internals of how the Oracle database handles transactions. In this case the communication mechanism of foreground sessions to the logwriter process is examined. The tests in this article have been executed using the following versions:
– Oracle database 12.1.0.2.161018
– Oracle linux 7.2, kernel 4.1.12-61.1.14.el7uek.x86_64 (UEK4)

martin.bach's picture

Building an RPM for the Oracle database on Oracle Linux 7

Thinking about automation a lot, especially in the context of cloud computing, I have decided to create a small series of related posts that hopefully help someone deploying environments in a fully automated way. As my colleague @fritshoogland has said many times: the only way to deploy database servers (or any other server for that matter) in a consistent way, is to do it via software. No matter how hard you try to set up 10 identical systems manually, there will be some, maybe ever so subtle, differences between them. And with the cloud you probably have 42 systems or more to deal with! In this context, my first post could be a building block: the provisioning of the Oracle RDBMS the form of an RPM.

The idea

In a nutshell, I would like to

fritshoogland's picture

A technical security analysis of the snmp daemon on Exadata

Recently I was asked to analyse the security impact of the snmp daemon on a recent Exadata. This system was running Exadata image version 12.1.2.1.3. This blog article gives you an overview of a lot of the things that surround snmp and security.

First of all what packages are installed doing something with snmp? A list can be obtained the following way:

# rpm -qa | grep snmp
net-snmp-utils-5.5-54.0.1.el6_7.1.x86_64
net-snmp-libs-5.5-54.0.1.el6_7.1.x86_64
net-snmp-5.5-54.0.1.el6_7.1.x86_64
sas_snmp-14.02-0103.x86_64

Essentially the usual net-snmp packages and a package called ‘sas_snmp’.

A next important thing is how the firewall is configured. However, the default setting of the firewall on the compute nodes with exadata is the firewall turned off:

fritshoogland's picture

Transactions and SCNs

It’s general knowledge that the Oracle database is ACID compliant, and that SCNs or ‘system change numbers’ are at the heart of this mechanism. This blogpost dives into the details of how the Oracle engine uses these numbers.

Oracle database version 12.1.0.2.161018
Operating system version: OL 7.2, kernel: 4.1.12-61.1.14.el7uek.x86_64 (UEK4)

Redo generation
Whenever DML is executed, redo is generated in the form of ‘change vectors’. These change vectors are copied into the redo buffer as part of the transaction, during the transaction. The function that performs this action is called ‘kcrfw_copy_cv()’. This can be derived by watching the foreground process perform memory copy into the memory area of the redo buffer.

In order to do this, you first need to find the memory area of the redo buffer. This can be done by executing ‘oradebug setmypid’ and ‘oradebug ipc’ as sysdba, and examine the resulting trace file:

fritshoogland's picture

Watching is in the eye of the beholder

Recently I was investigating the inner working of Oracle. One of the things that is fundamental to the Oracle database is the SCN (system change number). SCNs are used to synchronise changes in the database. There is one source for SCNs in every instance (kcbgscn; the global or current SCN in the fixed SGA), and there are multiple tasks for which Oracle keeps track of synchronisation using SCNs. A few of these tasks for which Oracle stores and uses SCNs to keep track of progress are on disk SCN and lwn SCN.

This blogpost is about some oddities I found when using gdb (the GNU debugger) to watch memory locations of a running Oracle database. This should not be done on a production instance, and is purely for research purposes. Only use the methods mentioned in this article if you are absolutely sure what you are doing, and/or if you are using an Oracle instance that can be crashed and can be restored.

martin.bach's picture

Little things worth knowing: when a transient ASM disk failure cannot be fixed in time

In the previous blog post I used libvirt and KVM in my lab environment to simulate a transient disk failure and how to recover from it. This post takes this example a step further: I am simulating another disk failure, but this time won’t pretend I can fix the issue and put it back. In other words, I simulate the effect of the disk_repair_time hitting zero.

Most of what I am covering here is an extension of the previous post, I’ll mention the main detail here for your benefit, but would like to invite you to revert to the previous post for more detail.

The idea is to show you the output of the ASM alert.log and result of the lost disk in the V$-views.

As with the previous post, the code examples in this one are for demonstration purposes only!

martin.bach's picture

Little things worth knowing: transient ASM disk failures in 12c

For quite some time we have been treated nicely by ASM when it comes to transient disk failures. Since 11.1 (if memory serves me right), transient failures won’t cause an ASM disk to be dropped immediately. This is good, because it can potentially save a lot of time! When a disk is dropped from an ASM disk, a rebalance operation is unavoidable. And there is more than a chance of another one following it, because ultimately you want to add the disk back to the disk group. Well, to be fair, this is only applicable for normal or high redundancy disk groups.

The feature I just described very briefly is referred to as ASM Fast Mirror Resync, and documented in the ASM Administrator’s Guide in section Administering Oracle ASM Disk Groups.

pete.sharman's picture

Installing a 12.1.0.5 agent on a Linux 5.x platform from a 13.1 OMS

Is that enough version numbers for you? </p />
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fritshoogland's picture

How much memory is truly used by my Oracle instance?

There are many posts about the amount of memory that is taken by the Oracle database executables and the database SGA and PGA. The reason for adding yet another one on this topic is a question I recently gotten, and the complexities which surrounds memory usage on modern systems. The intention for this blogpost is to show a tiny bit about page sharing of linux for private pages, then move on to shared pages, and discuss how page allocation looks like with Oracle ASMM (sga_target or manual memory).

The version of linux in this blogpost is Oracle Linux 7.2, using kernel: 4.1.12-37.6.3.el7uek.x86_64 (UEK4)
The version of the Oracle database software is 12.1.0.2.160719 (july 2016).

glennfawcett's picture

MemSQL — my next adventure

I have been working with Oracle database performance for many years now, but for me it is time to make a change.  Today I start with an exciting new and nimble company MemSQL.

MemSQL offers a new approach to real time analytics with lock-free data structures, in-memory row store, and column store.   MemSQL can perform Real-Time analysis as well as pull data from traditional EDW or Hadoop.

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