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How long does a logical IO take?

This is a question that I played with for a long time. There have been statements on logical IO performance (“Logical IO is x times faster than Physical IO”), but nobody could answer the question what the actual logical IO time is. Of course you can see part of it in the system and session statistics (v$sysstat/v$sesstat), statistic name “session logical reads”. However, if you divide the number of logical reads by the total time a query took, the logical IO time is too high, because then it assumed all the time the query took was spend on doing logical IO, which obviously is not the case, because there is time spend on parsing, maybe physical IO, etc. Also, when doing that, you calculate an average. Averages are known to hide actual behaviour.

fsfreeze in Linux

The fsfreeze command, is used to suspend and resume access to a file system. This allows consistent snapshots to be taken of the filesystem. fsfreeze supports Ext3/4, ReiserFS, JFS and XFS.

A filesystem can be frozen using following command:

#cccccc;line-height: 1.4"># /sbin/fsfreeze -f /data

Now if you are writing to this filesystem, the process/command will be stuck. For example, following command will be stuck in D (UNINTERUPTEBLE_SLEEP) state:

#cccccc;line-height: 1.4"># echo “testing” > /data/file

Only after the filesystem is unfreezed using the following command, can it continue:

Quick Tip : Oracle User Ulimit Doesn’t Reflect Value on /etc/security/limits.conf

So the other day I was trying to do a fresh installation of a new Oracle EM12cR4 in a local VM,  and as I was doing it with the DB 12c, I decided to use the Oracle preinstall RPM to ease my installation of the OMS repository database. Also I was doing both the repository and EM12c OMS install in the same VM, that is important to know.

[root@em12cr4 ~]# yum install oracle-rdbms-server-12cR1-preinstall -y

I was able to install the DB without any issues, but when I was trying to do the installation of EM12cR4, an error in the pre-requisites popped up:

WARNING: Limit of open file descriptors is found to be 1024.

For proper functioning of OMS, please set “ulimit -n” to be at least 4096.

And if I checked the soft limit for the user processes , it was set to 1024:

Little things worth knowing: Is there a penalty in establishing a connection to Oracle using the MAA connection string? Part 2

In the very lengthy previous post about the MAA connect string I wanted to explain the use of the MAA connection string as promoted by Oracle. I deliberately kept the first part simple: both primary and standby cluster were up, and although the database was operating in the primary role on what I called standby cluster (again it’s probably not a good idea to include the intended role in the infrastructure names) there was no penalty establishing a connection.

Little things worth knowing: Is there a penalty in establishing a connection to Oracle using the MAA connection string?

Sorry for the long title!

I had a question during my session about “advanced RAC programming features” during the last Paris Oracle Meetup about the MAA connection string. I showed an example taken from the Appication Continuity White Paper ( Someone from the audience asked me if I had experienced any problems with it, such as very slow connection timeouts. I haven’t, but wanted to double-check anyway. This is a simplified test using a sqlplus connection since it is easier to time than a call to a connection pool creation. If you know of a way to reliably do so in Java/UCP let me know and I’ll test it.

Little things worth knowing: what does opatchauto actually do?

I recently applied system patch 20132450 to my installation on a 2 node RAC system on Oracle Linux 7.1. While ensuring that OPatch is the latest version available I came across an interesting command line option in opatchauto. It is called “-generateSteps”.

Hey dude, where’s my memory? part 2

In my previous article I started exploring the memory usage of a process on a recent linux kernel (2.6.39-400.243.1 (UEK2)), recent means “recent for the Enterprise Linux distributions” in this context, linux kernel developers would point out that the kernel itself is at version 3.19 (“stable version” at the time of writing of this blogpost).

Hey dude, where’s my memory?

This blogpost is about finding the actual amount of memory a process is taking. In order to do so, this post dives into the memory mechanisms of Linux. The examples in this article are taken from an Oracle Linux version 6.6 server, with kernel 2.6.39-400.243.1 (UEK2). This is written with the Oracle database processes in mind, but actually uses examples of a processes running ‘cat’, which means the contents of this post are absolutely not limited to Oracle database processes.

Let’s start off with a simple example. Let’s look at our own memory map. In order to do so, I use the ‘cat’ executable and the ‘maps’ entry in the proc pseudo-filesystem. This is how that is done, including the result: