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:

# lsblk
NAME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
fd0           2:0    1    4K  0 disk
sda           8:0    0    (more...)

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 (more...)

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 (more...)

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;
begin
        return l_value+1;
end;
/
-- A SQL statement that uses the PL/SQL function
select sum(add_one(id))  (more...)

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 (more...)

Oracle 12 and latches, part 3

This post is about manually calling and freeing a shared latch. Credits should go to Andrey Nikolaev, who has this covered in his presentation which was presented at UKOUG Tech 15. I am very sorry to see I did miss it.

Essentially, if you follow my Oracle 12 and shared latches part 2 blogpost, which is about shared latches, I showed how to get a shared latch:

SQL> oradebug setmypid
Statement processed.
SQL> oradebug  (more...)

Oracle 12c in-memory option and IO

This article is about the Oracle 12c in-memory option, and specifically looks at how the background worker processes do IO to populate the in-memory column store.

Hardware: Apple Macbook with VMWare Fusion 7.1.3.
Operating system: Oracle Linux 6.7, kernel: 3.8.13-118.el6uek.x86_64.
Database version: Oracle 12.1.0.2
Patch: opatch lspatches
19392604;OCW PATCH SET UPDATE : 12.1.0.2.1 (19392604)
19303936;Database Patch Set Update : 12.1. (more...)

Direct path and buffered reads again: compressed tables.

In the previous post on the decision between buffered and direct path reads I showed the decision is depended on the version. Up to and including version 11.2.0.2 the size of a segment needs to be five times small table threshold in order to be considered for direct path reads, and starting from 11.2.0.3 the database starts considering direct path reads starting from small table threshold. The lower limit (more...)

Direct path and buffered reads again.

Since the direct path feature for serial processes was discovered after it became available in Oracle 11.2.0.1 (as far as I know, I haven’t checked Oracle 11.1), there have been a lot of blog posts on when this happens. A lot of these do not specify the Oracle version, which is a failure in my opinion. There are different decisions made in different versions.

The purpose of this blogpost is to show (more...)

How the log writer and foreground processes work together on commit.

(warning: this is a rather detailed technical post on the internal working of the Oracle database’s commit interactions between the committing foreground processes and the log writer)

After the Trivadis Performance days I was chatting to Jonathan Lewis. I presented my Profiling the log writer and database writer presentation, in which I state the foreground (user/server) process looks at the commit SCN in order to determine if its logbuffer contents are written to disk by (more...)