martin.bach's picture

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.


Hotsos 2016 Recap

I wanted to go to Hotsos Symposium for quite some time, having heard so many great topics from there for years. And every time I was a bit lazy to think about what I can talk about. Apparently I thought that everything I know is well covered elsewhere, so why would I be accepted. Plus all these complexities of budget, getting a visa, travel arrangements, jet lag, and personal matters in between. Last year, when call for papers was still open, I realized that there’s a good chance I can make it to Hotsos in 2016: I had a budget, visa is a doable thing, and most importantly I knew I had a topic to talk about which most likely will not be in competition with other speakers.

Jonathan Lewis's picture

Cursor_Sharing problem

Here’s a possible bug (though maybe “not a bug”) that came up over the weekend on the OTN database forum. An application generating lots of “literal string” SQL was tested with cursor_sharing set to force. This successfully forced the use of bind variable substitution, but a particular type of simple insert statement started generating very large numbers of child cursors – introducing a lot of mutex waits and library cache contention. Here’s a (substituted) statement that was offered as an example of the problem:

Jonathan Lewis's picture


My favourite format options for dbms_xplan.display_cursor().

This is another of those posts where I tell you about something that I’ve frequently mentioned but never documented explicitly as a good (or, at least, convenient) idea. It also another example of how easy it is to tell half the story most of the time when someone asks a “simple” question.

dbakevlar's picture

Database Comparisons Using AWR Warehouse Part 1- ADDM Comparison

A lot of my ideas for blog posts come from questions emailed to me or asked via Twitter.  Today’s blog is no different, as I was asked by someone in the community what the best method of comparing databases using features within AWR when migrating from one host and OS to another.

Jonathan Lewis's picture

RI Locks

RI = Referential Integrity: also known informally as parent/child integrity, and primary (or unique) key/foreign key checking.

I’m on a bit of a roll with things that I must have explained dozens or even hundreds of times in different environments without ever formally explaining them on my blog. Here’s a blog item I could have done with to response to  a question that came up on the OTN database forum over the weekend.

What happens in the following scenario:

Jonathan Lewis's picture

E-rows / A-rows

A recent post on the OTN database forum reminded me how easy it is to forget to keep repeating a piece of information after the first couple of hundred times you’ve explained it. No matter how “intuitively obvious” it is for one person, it’s new to someone else.

Here’s an execution plan that raised the question that prompted this note – it comes from calling dbms_xplan.display_cursor() with the ‘allstats last’ format option after enabling rowsource execution statisics (using hint gather_plan_statistics, or setting parameter statistics_level to all, or setting hidden parameter “_rowsource_execution_statistics” to true):

dbakevlar's picture

EM13c- Applying System Patches with the OMS Patcher

The OMS Patcher is a newer patching mechanism for the OMS specifically, (I know, the name kind of gave it away…)  Although there are a number of similarities to Oracle’s infamous OPatch, I’ve been spending a lot of time on OTN’s support forums and via email, assisting folks as they apply the first system patch to

Jonathan Lewis's picture


The OTN database forum supplied a little puzzle a few days ago – starting with the old, old, question: “Why is the plan with the higher cost taking less time to run?”

The standard (usually correct) answer to this question is that the optimizer doesn’t know all it needs to know to predict what’s going to happen, and even if it had perfect information about your data the model used isn’t perfect anyway. This was the correct answer in this case, but with a little twist in the tail that made it a little more entertaining. Here’s the query, with the two execution plans and the execution statistics from autotrace:

iggy_fernandez's picture

When SQLT is not enough

A SQLT report has all kinds of pertinent information including—to name just a few—optimizer settings, indexes, statistics, plan history, and view definitions. However, sometimes a SQLT report is not enough to solve a SQL performance problem.(read more)

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