A server process is waiting on “db file sequential read” wait event after it performs a single block I/O operation and it is waiting for the operating system to complete it.

A single block I/O operation occurs when a server process request a single block for the operating system. This happens during operations such as index unique or range scans, table access by rowid, etc.

Seeing this wait event in the top wait events is expected and doesn’t mean that something is wrong. Remember, users are working and reading indexes or table blocks from the disk is common. However, if we see that this event is very high and consuming unreasonable time, we need to check our SQL statement, they might need tuning, or might have unnecessary hints such as the “index” hint.

To read about analyzing wait events in an AWR report, see my post analyzing oracle awr reports – top 5 events.

A server process is waiting on “db file scattered read” wait event after it performs a multiblock I/O operation and it is waiting for the operating system to complete it.

A multiblock I/O operation occurs when the server process asks for a “chunk” of blocks and not for a single block. The amount of blocks in the “chunk” is determined by the DB_FILE_MULTIBLOCK_READ_COUNT initialization parameter. For example, if the database block size is 8k and DB_FILE_MULTIBLOCK_READ_COUNT is 8, a single multiblock I/O operation will read 8 blocks, which is 64KB of data. The maximum size of a multiblock I/O operation is determined by the operating system and storage.

When talking about performance, there are several things we need to remember:

• A server process can perform multiblock I/O operations only in certain cases like full table scans and fast full index scans. It is important to remember that multiblock I/O and DB_FILE_MULTIBLOCK_READ_COUNT are irrelevant to other operations such as index  range/unique scans.
• Setting the DB_FILE_MULTIBLOCK_READ_COUNT to a high value will result in less I/O operations (as long as it is not exceeding the operating system and storage limitation).
• Setting the DB_FILE_MULTIBLOCK_READ_COUNT to a high value can also result in unwanted full table scans, since a higher value lowers the optimizer cost of full table scans.
• Starting in 10gR2, Oracle will determine the best value for the DB_FILE_MULTIBLOCK_READ_COUNT parameter if it is not set. If the parameter is set, Oracle will not automatically set its value.

To read about analyzing wait events in an AWR report, see my post analyzing oracle awr reports – top 5 events.

From Oracle documentation:
“Wait events are statistics that are incremented by a server process or thread to indicate that it had to wait for an event to complete before being able to continue processing.”

For example, wait events can be related to I/O, locks, memory allocation, etc.

The top events tell us what the server processes wait for the most. Eliminating these wait events will definately reduce execution time, since server processes will not have to wait this time for other operations to complete.

There are many wait event in Oracle databases, this is a list of some important and common wait events:

• buffer busy waits
• free buffer waits
• db file scattered read
• db file sequential read
• enqueue waits
• log buffer space
• log file sync

To identify a performance problem we will check the time spent on each wait event and the average wait time. These numbers can be very different for different wait events, I/O wait events will probably have a low average, while waiting on locking related wait events can take a long time. Don’t forget to compare the wait time to the AWR report time period, waiting on I/O for an hour may be reasonable if the AWR report was generated on 6 hours period, but may indicate a problem on a 10 minutes report (the waiting time can exceed the total report time, since it is cumulative for all server processes).

The best way to find performance problems is to compare the wait events and the time spent on them to an AWR report from a time period in which the database worked fine. So try to save reports as a baseline for future reference.

In the next posts we’ll go deeper into analyzing the rest of the AWR report.

Sometimes, all we need is to get a value from an index we have, and this index is not a sequence number, but a number that has meaning or even text. To solve this problem we can use a regular table or temporary table, but these have some overhead and can be affected by other users or operations.

A simple example for this scenario can be coloring the world map in 4 colors (red, green, blue and yellow). In this example each country has a color and we need to know the color for a given country and go over the countries to check if they have a color.

The most effective way to do that is to use associative arrays with varchar2(30) as the index (for the country names) and varchar2(6) and the value (for the colors).

The following code shows how to work with associative arrays, add new cells, check the array length and go over the entire array.

<pre>declare
   type map_array is table of varchar2(4) index by varchar2(30);
   map map_array;
   idx varchar2(30);
begin
   -- add countries to the array
   map('France'):='blue';
   map('Italy'):='red';

   -- print the number of countries in the arrary
   dbms_output.put_line('There are '||map.count||' countries');

   -- go over the entire array
   idx:=map.first;
   while (idx is not null)
   loop
      dbms_output.put_line('Country '||idx||' is '||map(idx));
      idx:=map.next(idx);
   end loop;

   -- check if a country exists
   idx:='England';
   if (map.exists(idx))
   then
      dbms_output.put_line(idx||' has the color '||map(idx));
   else
      dbms_output.put_line(idx||' does not have a color');
   end if;
end;

Let’s take an example where bit can be very useful. An example can be restaurant website, where we need to keep information about the type of food in the restaurants. There are several ways to keep this information (column for each type and 0 or 1 for true and false, or a separate one-to-many table that contains a rows which are restaurant id and food type couple), but we can also use bits to do that. We can decide that Italian food will be the first bit, steakhouse will be the second, pizza is the third and so on. From that we can generate a number that matches the food types in the restaurant. For example, with the type we mentioned, a Italian restaurant that has also pizza will be 101 in binary which is the number 5. A steak house will be 010 which is 2.

This solution requires a very little storage, and it is very easy to handle in the application.

The only problem is that we may need bitwise operators in Oracle for some queries. Oracle supplies the BITAND function, but does not supply any other bitwise functions. We will have to create these for ourselves:

BITOR

create or replace function bitor (x number, y number)
return number
is
begin
   return (x+y)-bitand(x,y);
end;

BITXOR

<pre>create or replace function bitxor (x number, y number)
return  number
is
begin
   return bitor(x,y)-bitand(x,y);
end;

Information about the sort segments can be found in V$SORT_SEGMENT dictionary view. This view contains one row for each sort segment and the following columns:

TOTAL_EXTENTS – The number of extents allocated to the sort segment

TOTAL_BLOCKS – The number of blocks allocated to the sort segment

USED_EXTENTS – number of extents that are currently being used

USED_BLOCKS – number of blocks that are currently being used

FREE_EXTENTS – number of extents that are allocated to the segment but currently marked as free

FREE_BLOCKS – number of blocks that are allocated to the segment but currently marked as free

You can find information about usage of the temporary tablespace in the V$SORT_USAGE dictionary view. This view contains the following columns:

USERNAME – database user name

SESSION_ADDR – address of the session, can be used to identify the session in V$SESSION according to the SADDR column

SQL_ID – the identifier of the SQL that requires the sort or join, can be used to identify the SQL from V$SQL according to the SQL_ID column

EXTENTS – number of extents in the temporary segment being used by this session

BLOCKS – number of blocks in the temporary segment being used by this session

Then came Oracle 11g with the “active standby database” feature (sometimes referenced as real-time query).

With this feature we can open the database in “read only” mode, and can still apply archive logs from the primary.

Let’s see how it works…

If we knew where is the last used block in the file, we would know the minimum size of the datafile.

Use this query to get the file’s high water mark:

select /*+ rule */
a.tablespace_name,
a.file_name,
a.bytes/1024/1024 file_size_MB,
(b.maximum+c.blocks-1)*d.block_size/1024/1024 highwater
from
dba_data_files a,
(select file_id,max(block_id) maximum
from dba_extents
group by file_id) b,
dba_extents c,
    dba_tablespaces d
where a.file_id  = b.file_id
and c.file_id  = b.file_id
and c.block_id = b.maximum
  and d.tablespace_name=a.tablespace_name
order by a.tablespace_name,a.file_name;

The query returns the following columns:

TABLESPACE_NAME - the name of the tablespace

FILE_NAME - the name of the datafile

FILE_SIZE_MB - the current size of the file (in MB)

HIGHWATER - the minimum size that the file can reduced to (in MB)

When installing Oracle on Linux, there are several prerequisites steps that needs to be completed before the installation process ifself.
The installation GUI is identical to other operating systems, but the steps we need to complete before starting the installation are different.
This video will show you how to install Oracle 11gR1, step by step.

This video shows Oracle 11gR1 installation on Linux Cent OS 5 (which is similar to RedHat 5), but can be used to understand the general Oracle installation on Linux.

You can download the Oracle Database software from OTN:

• Oracle Database 11g Release 1- OTN

We will start by checking the requirements for Oracle installation.

1)    Checking the memory, SWAP, Kernel version and free space requirements.

Memory – Oracle 11g requires at least 1GB RAM.
You’ll be able to check the server’s memory by executing the command:

[root ~]grep MemTotal /proc/meminfo

Swap requirements are specified in the documentation and based on the RAM.
Check the server’s SWAP by executing the command:

[root ~]grep SwapTotal /proc/meminfo

At least 400Mb are needed in /tmp directory. In addition, Oracle 11g software requires about 3.5GB.
Check the /tmp available space by executing the command:

[root ~]df -k /tmp

Check the server’s available space by executing the command:

[root ~]df -k

The available column will display the free space in kilobytes.

The OS kernel version should be 2.6.18 or higher.
Check the kernel version by executing the command:

[root ~] cat /proc/version
OR
[root ~] uname –r

2)     Required Packages

Oracle 11g software needs several rpm packages to be installed on the server.
You can find the full list of packages in the installation guide:
http://download.oracle.com/docs/cd/B28359_01/install.111/b32002/pre_install.htm#CHDHFGBJ

The command “rpm -qa” prints the list of all installed packages. With “grep” command we will look for specific packages.

Example: to verify that the gcc package is install, use:

[root ~]rpm –qa | grep gcc

3)    Creating users and groups.

We will create two new groups: ‘dba’ and ‘oinstall’ and the ‘oracle’ user.

[root ~] groupadd oinstall
[root ~] groupadd dba
[root ~] useradd -g oinstall -G dba oracle
[root ~] passwd oracle

4)    Setting Kernel parameters.

Open the /etc/sysctl.conf file  and configure the following kernel parameters:

fs.file-max = 6815744
kernel.shmall = 2097152
kernel.shmmax = 2147483648
kernel.shmmni = 4096
kernel.sem = 250 32000 100 128
net.ipv4.ip_local_port_range = 9000 65500
net.core.rmem_default = 262144
net.core.rmem_max = 4194304
net.core.wmem_default = 262144
net.core.wmem_max = 1048576

Additional information about the kernel parameter can be found in the Documentation:
http://download.oracle.com/docs/cd/B28359_01/install.111/b32002/pre_install.htm#BABBBDGA

After editing the file, execute the following command to apply the changes.

[root ~] sysctl -p

5)     Add the following lines to the /etc/security/limits.conf file:

oracle              soft    nproc    2047
oracle              hard    nproc   16384
oracle              soft    nofile  1024
oracle              hard    nofile  65536

6)    Add the following line to the /etc/pam.d/login file:

session   required   pam_limits.so

7)    Add the following lines to the /etc/profile file: This section sets the soft limit for user ‘oracle’ upon login:

if [ $USER = "oracle" ]; then
 if [ $SHELL="/bin/ksh" ] ; then
    ulimit -p 16384
    ulimit -n 65536
 else
    ulimit -p 16384 -n 65536
 fi
fi

8)    Creating directories for the Oracle installation: We need to create the directory for Oracle installation. This directory should be owned by user ‘oracle’

[root ~]mkdir -p /u01/app/oracle/product/11.1.0/db_1
[root ~]chown -R oracle:oinstall /u01
[root ~]chmod -R 775 /u01

9)    Disable access control:

Access control does not allow opening X-Windows from any server. We will use xhost command to disable this access control.

[root ~] xhost +

10)   Edit the /etc/profile file:

We will now connect as user ‘oracle’ and configure the environment in the bash_profile file.

[root ~]su – oracle
[oracle ~]vi .bash_profile

We will set the ORACLE_BASE for oracle base directory and ORACLE_SID environment variables. Add the following lines to bash_profile file:

export ORACLE_HOSTNAME=<SERVER_NAME>
export ORACLE_BASE=/u01/app/oracle;
export ORACLE_HOME=$ORACLE_BASE/product/11.1.0/db_1;
export ORACLE_SID=orcl;

Apply the changes so that the new variables will be set:

[oracle ~] . .bash_profile

The pre-requirement part is completed. All we have to do now is to start the installation.

11)    Navigate to the installation directory and start the installation:

[oracle ~] cd /install/database
[oracle ~] ./runInstaller

We will skip the installation part as it is quite straightforward, and go to the end of the installation part.

12)    Execute configuration scripts as root user:

At the end of the installation, you’ll see the “execute configuration scripts” wizard window. We should run these scripts as root. Open a Terminal as root and execute the below scripts”

[root ~] /u01/app/oracle/oraInventory/orainstRoot.sh
[root ~] /u01/app/oracle/product/11.1.0/db_1/root.sh

The installation is now complete.

13)    Updating the .bash_profile file with more environment variables:

The last thing we need to do is updating the .bash_profile file:

[oracle ~] vi .bash_profile

We will set the ORACLE_HOME for oracle home directory and the path environment variables. Add the following lines to bash_profile file:

export ORACLE_HOSTNAME=<SERVER_NAME>
export ORACLE_BASE=/u01/app/oracle;
export ORACLE_HOME=/u01/app/oracle/product/11.1.0/db_1;
export ORACLE_SID=orcl;
export PATH=/usr/sbin:$PATH;
export PATH=$PATH:$HOME/bin:$ORACLE_HOME/bin

Apply the changes so that the new variables will be set:

[oracle ~]. .bash_profile

14) Verify that everything is configured correctly.

[oracle ~] sqlplus / as sysdba

SQL*Plus: Release 11.1.0.1.0 Production on Mon Oct 25 12:26:26 2010

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

Connected to:

Oracle Database 11g Enterprise Edition Release 11.1.0.1.0 - 64bit Production

With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL>exit