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Oracle database internals

Inmemory: Not all inmemory_size is usable to store tables.

I have been testing the inmemory column store product extensively and the product is performing well for our workload. However, I learnt a bit more about inmemory column store and I will be blogging a few them here. BTW, I will be talking about internals of inmemory in Oaktable world presentation, if you are in the open world 2014, you can come and see my talk: http://www.oraclerealworld.com/oaktable-world/agenda/

inmemory_size

inmemory area is another sub-heap of the top-level SGA heap

I blogged earlier about heap dump shared pool heap duration and was curious to see how the inmemory – 12.1.0.2 new feature – is implemented. This is a short blog entry to discuss the inmemory area heap.

Parameters

I have set the initialization parameters sga_target=32G and inmemory_size=16G, meaning, out of 32GB SGA, 16GB will be allocated to inmemory area and the remaining 16GB will be allocated to the traditional areas such as buffer_cache, shared_pool etc. I was expecting v$sgastat view to show the memory allocated for inmemory area, unfortunately, there are no rows marked for inmemory area (Command “show sga” shows the inmemory area though). However, dumping heapdump at level 2 shows that the inmemory area is defined as a sub-heap of the top level SGA heap. Following are the commands to take an heap dump.

How to reformat corrupt blocks which are not part of any segment?

There was a question in . Problem is that there were many corrupt blocks in the system tablespace not belonging to any segment. Both DBV and rman throws errors, backup is filling the v$database_block_corruption with numerous rows. OP asked to see if these blocks can be reinitialized. Also, note 336133.1 is relevant to this issue on hand.

Data visualization, px qref waits, and a kernel bug!

Data visualization is a useful method to identify performance patterns. In most cases, I pull custom performance metrics from AWR repository and use tableau to visualize the data. Of course, you can do the visualization using excel spreadsheet too.

Problem definition
We had huge amount of PX qref waits in a database:

Scripts to create AWR reports quickly.

It is easier to create one or two AWR reports quickly using OEM. But, what if you have to create AWR reports for many snapshots? For example, your Oracle support analyst wants you to supply 10 1-hour AWR reports from 10AM to 8PM in a 8 node cluster? That’s about 80 AWR reports to create! Okay, okay, I may(!) be overselling it, but you get the point. It is useful to have a script to create AWR report for all instances for a given range of snapshot IDs. Following scripts are handy:

Oaktable world presentation

I will be hacking RAC internals with few LINUX tools in Oaktable world presentation series, in SFO. Details are available at Oaktable World 2013

Hope to see you there!

RAC Internals: cached sequences and 12c

Introduction

I blogged about DFS lock handle contention in an earlier blog entry. SV resources in Global Resource Directory (GRD) is used to maintain the cached sequence values. I will further probe the internal mechanics involved in the cached sequences. I will also discuss minor changes in the resource names to support pluggable databases (version 12c).

SV resources

Let’s create an ordered sequence in rs schema and then query values from the sequence few times.

create sequence rs.test_seq order cache 100;
select rs.test_seq.nextval from dual; -- repeated a few times.
...
/
21

Sequence values are permanently stored in the seq$ dictionary table. Cached sequence values are maintained in SV resources in GRD and SV resource names follows the naming convention to include object_id of the sequence. I will generate a string using a small helper script and we will use that resource name to search in the GRD.

Book: Expert Oracle RAC 12c

A quick note, Expert Oracle RAC book co-written by me is available now: Expert Oracle RAC 12c. I have written about 6 chapters covering the RAC internals that you may want to learn :) I even managed to discuss the network internals in deep, after all, network is one of the most important component of a RAC cluster.

Dude, where is my redo?

This blog entry is to discuss a method to identify the objects inducing higher amount of redo. First,we will establish that redo size increased sharply and then identify the objects generating more redo. Unfortunately, redo size is not tracked at a segment level. However, you can make an educated guess using ‘db block changes’ statistics. But, you must use logminer utility to identify the objects generating more redo scientifically.

Detecting redo size increase

AWR tables (require Diagnostics license) can be accessed to identify the redo size increase. Following query spools the daily rate of redo size. You can easily open the output file redosize.lst in an Excel spreadsheet and graph the data to visualize the redo size change. Use pipe symbol as the delimiter while opening the file in excel spreadsheet.

Clusterware Startup

The restart of a UNIX server call initialization scripts to start processes and daemons. Every platform has a unique directory structure and follows a method to implement server startup sequence. In Linux platform (prior to Linux 6), initialization scripts are started by calling scripts in the /etc/rcX.d directories, where X denotes the run level of the UNIX server. Typically, Clusterware is started at run level 3. For example, ohasd daemon started by /etc/rc3.d/S96ohasd file by supplying start as an argument. File S96ohasd is linked to /etc/init.d/ohasd.

S96ohasd -> /etc/init.d/ohasd

/etc/rc3.d/S96ohasd start  # init daemon starting ohasd.

Similarly, a server shutdown will call scripts in rcX.d directories, for example, ohasd is shut down by calling K15ohasd script: