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Experimenting with the ZFSSA’s snapshot capability using the simulator

Recently I have been asked how the Copy-on-Write cloning works on the ZFS Storage Appliance. More specifically, the question was about the “master” copy: did it have to be static or could it be rolled forward? What better than a test to work out how it works. Unfortunately I don’t have an actual system available to me at home so I had to revert to the simulator, hoping that it represents the real appliance accurately.


First I downloaded the ZFS Storage Appliance Simulator from the Oracle website and created a nice, new, shiny storage system (albeit virtual). Furthermore I have an Oracle Linux 7 system with UEK3 that will attach to the ZFSSA using dNFS. The appliance has an IP address of while the Linux system is accessible via This is of course a virtual toy environment, a real life setup would be quite different using IPMP and multiple paths preferably over Infiniband.


Trouble with multiple SCAN listeners in 12c Release 1

Prompted by comments made by readers about my posts describing how to add a second SCAN in 12c Release 1 and problems regarding the listener_networks parameter I thought it was time to revisit the situation. I’m still running Oracle Linux 7.1/UEK3 (although that should not matter) but upgraded to This is the release that is reportedly showing the odd behaviour. I don’t recall my exact version when I wrote the original posts back in April 2014, but by looking at them I think it all worked ok at the time. Here is my current patchlevel after the upgrade to the troublesome PSU.

Presenting “UNIX/Linux Tools For The Oracle DBA” at #C16LV

Going back to the invention of the graphical user interface (GUI) in the 1970s, there has been tension between the advocates of the magical pointy-clickety GUI and the clickety-clackety command-line interface (CLI).

Part of it is stylistic… GUI’s are easier, faster, more productive.

Part of it is ego… CLI’s require more expertise and are endlessly customizable.

Given the evolutionary pressures on technology, the CLI should have gone extinct decades ago, as more and more expertise is packed into better and better GUI’s.  And in fact, that has largely happened, but the persistence of the CLI can be explained by four persistent justifications…

Oracle Linux 7 disks and udev

I was setting up disk devices for ASM in Oracle Linux 7. I knew things have changed between Oracle Linux 6 and Oracle Linux 7, but only just a little bit.

First of all, let’s take a look at the current disk usage. To see what disk devices are visible and how they are used, use lsblk:

The Oracle wait interface granularity of measurement

The intention of this blogpost is to show the Oracle wait time granularity and the Oracle database time measurement granularity. One of the reasons for doing this, is the Oracle database switched from using the function gettimeofday() up to version 11.2 to clock_gettime() to measure time.

This switch is understandable, as gettimeofday() is a best guess of the kernel of the wall clock time, while clock_gettime(CLOCK_MONOTONIC,…) is an monotonic increasing timer, which means it is more precise and does not have the option to drift backward, which gettimeofday() can do in certain circumstances, like time adjustments via NTP.

The first thing I wanted to proof, is the switch of the gettimeofday() call to the clock_gettime() call. This turned out not to be as simple as I thought.

PL/SQL context switch, part 2

This is the second blogpost on using PL/SQL inside SQL. If you landed on this page and have not read the first part, click this link and read that first. I gotten some reactions on the first article, of which one was: how does this look like with ‘pragma udf’ in the function?

Pragma udf is a way to speed up using PL/SQL functions in (user defined function), starting from version 12. If you want to know more about the use of pragma udf, and when it does help, and when it doesn’t, please google for it.

create or replace function add_one( value number ) return number is
        pragma udf;
        l_value number(10):= value;
        return l_value+1;

select sum(add_one(id)) from t2;

As you can see, really the only thing you have to do is add ‘pragma udf’ in the declaration section of PL/SQL.

PL/SQL context switch

Whenever you use PL/SQL in SQL statements, the Oracle engine needs to switch from doing SQL to doing PL/SQL, and switch back after it is done. Generally, this is called a “context switch”. This is an example of that:

-- A function that uses PL/SQL 
create or replace function add_one( value number ) return number is
        l_value number(10):= value;
        return l_value+1;
-- A SQL statement that uses the PL/SQL function
select sum(add_one(id)) from t2;

Of course the functionality of the function is superfluous, it can easily be done in ‘pure’ SQL with ‘select sum(id+1) from t2’. But that is not the point.
Also, I added a sum() function, for the sake of preventing output to screen per row.

Video: Oracle Linux Virtual Machine (VM) on Amazon Web Services (AWS)

Continuing the cloud theme, here is a quick run through of the process of creating an Oracle Linux virtual machine on Amazon Web Services (AWS).

A few months ago I wrote an article about installing an Oracle database on AWS.

I updated the images in that article last night to bring them in line with this video.

Video: Oracle Linux Virtual Machine (VM) on Micorosft Azure

The interface for Microsoft Azure has been re-jigged since I last did screen shots, so I did a run through of creating an Oracle Linux VM and recorded it for my channel.

I also updated the associated article.



Introducing stapflame, extended stack profiling using systemtap, perf and flame graphs

There’s been a lot of work in the area of profiling. One of the things I have recently fallen in love with is Brendan Gregg’s flamegraphs. I work mainly on Linux, which means I use perf for generating stack traces. Luca Canali put a lot of effort in generating extended stack profiling methods, including kernel (only) stack traces and CPU state, reading the wait interface via direct SGA reading and kernel stack traces and getting userspace stack traces using libunwind and ptrace plus kernel stack and CPU state. I was inspired by the last method, but wanted more information, like process CPU state including runqueue time.