Configure Rman Backupset Compression

RMAN compresses the backup set contents before writing them to disk. No extra uncompression steps are required during recovery when we use RMAN compression. RMAN has two types of compression:  

1.) Null Compression     and
2.) Unused Block Compression

1.) Null Compression :  When backing up datafiles into backup sets, RMAN does not back up the contents of data blocks that have never been allocated. This means RMAN will never backup the blocks that are ever used. For example: We have a tablespace having one datafile of size 100MB and out of 100MB only 50 MB is used. Then RMAN will backup only 50MB. 

2.) Unused Block Compression :  RMAN skips the blocks that do not currently contain data and this is called Unused Block Compression. RMAN now creates more compact backups of datafiles, by skipping datafile blocks that are not currently used to store data. No extra action is required on the part of the DBA to use this feature. 

Example :  We  have a tablespace having one datafile of size 100MB and out of 100MB, 50MB is used by the user tables. Then user dropped a table belonging to that tablespace which was of 25MB, with Unused Block Compression only 25MB of the files is backed up. In this example if null compression is used then it would have backed up 50MB because Null Compression will consider the blocks that are formatted/ever used. 

Binary Compression : Binary Compression can be done by specifying "AS COMPRESSED" clause in backup command, this compression is called as binary compression. RMAN can apply a binary compression algorithm as it writes data to backup sets. This compression is similar to the compression provided by many tape vendors when backing up data to tape. But we cannot give exact percentage of compression. This binary compression algorithm can greatly reduce the space required for disk backup storage. It is typically 2x to 4x, and greater for text-intensive databases.  The command to take the compressed backup :

RMAN> backup as compressed backupset database ;


There is no special command to restore database from the compressed backupsets. The restore command will be the same as with uncompressed backups.The restore from the compressed backpuset will take more time than uncompressed backupsets.
To use rman compression option, we can run the following RMAN commands to configure compression 


RMAN> CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO COMPRESSED BACKUPSET;
followed by ..
RMAN> CONFIGURE COMPRESSION ALGORITHM ‘HIGH’ ; 
or
RMAN> CONFIGURE COMPRESSION ALGORITHM ‘MEDIUM’ ; 
or
RMAN> CONFIGURE COMPRESSION ALGORITHM ‘LOW’ ; 
or
RMAN> CONFIGURE COMPRESSION ALGORITHM ‘BASIC’ ; 

Oracle 11g added several compression algorithms to compress data. They can be used for compressing tables, LOBs , compressed data pump exports or even RMAN backups. Unfortunately for some compression algorithms we need to purchase the “Advanced Compression Option”. The following table lists the available RMAN compression options, the most likely compression algorithm being used and states if an additional license is required:

The compression levels are BASIC, LOW, MEDIUM and HIGH and each affords a trade off related to backup throughput and the degree of compression afforded. If we have enabled the Oracle Database 11g Release 2 Advanced Compression Option, then we can choose from the following compression levels :

• HIGH - Best suited for backups over slower networks where the limiting factor is network speed
• MEDIUM - Recommended for most environments. Good combination of compression ratios and speed
• LOW - Least impact on backup throughput and suited for environments where CPU resources are the limiting factor.

Note:  The compression ratio generally increases from LOW to HIGH, with a trade-off of potentially consuming more CPU resources.

We can check the compression level by using the command .


SQL> select  *  from V$RMAN_COMPRESSION_ALGORITHM;
Output : 

I found a good scenario on net related to compression level having statistics about the this compression level . Here is the scenario : The environment being used was a freshly created 11g Release 2 database with some smaller tables in it. The total sum of all segments equals to 4.88 GB. All database data files excluding the temporary ones are 7.3 GB total. Excluding temporary and undo data files total size equates to 5.9 GB.


Here is the test results displays of the compression level :
Test results

As we  can see  from the  table  HIGH compression  does an  incredibly high  load on the  machine and  take extremely long but produces the smallest backup set size.Surprisingly BASIC compression (which is available without advanced compression license) does a good job as well and produces the second smallest backup set but takes nearly as long as doing uncompressed backups. But in other environment with faster CPUs this will change . 

In the test environment used either LOW or MEDIUM compression seems to be the best choice. Due to the fact MEDIUM produces a approx. 15% smaller backup set but taking only a few seconds more time to complete i would rank MEDIUM on 1st and LOW on second.

Finally we came to the conclusion that stronger the compression the smaller the backup size but the more CPU-intensive the backup is. If we do not have the advanced compression license BASIC compression will produce reasonable compression rates at moderate Load. If  we have the licence we have a lot more options to suit our needs.

If we want to test and optimize our rman backup, we basically have three major switches to play with :

• compression algorithmn
• rman parallelism and
• data transfer mechanism (SAN or Ethernet [this includes: iSCSI, NFS, CIFS, Backup to tape over Ethernet])

Enjoy     J J J

Oracle Advanced Compression

Oracle Advanced Compression and Oracle Database 11g Release 2 helps manage more data in a cost-effective manner. With data volumes, on average, tripling every two years, Oracle Advanced Compression delivers compression rates of 2-4x across all types of data and applications.storage savings from compression will cascade throughout the data center, reducing network traffic and data backups as well. And by reading fewer blocks off disk, Oracle Advanced Compression also improves query performance.

Oracle Advanced Compression is an option of the Oracle 11g database (separately licensed) that allows data in the database to be compressed. Oracle Advanced Compression offers the following advantages:

1.) OLTP Compression  : It allows structured and unstructured data to be compressed on insert,update and delete  operations.The following are features : 

• New compression algorithm uses deferred or batched approach
• Data is inserted as is without compression until  PCTFREE  value is reached.
• Compression of data starts once PCTFREE threshold is reached
• Can be enabled at  table, partition or tablespace level
• No need of decompressing the data during reads
• Recommended for low update activity tables

2.) Data Pump Compression   : In Data Pump, the compression of metadata was introduced in 10g and compression of "data" was introduced in 11g.This covers the following features : 

• Both are Inline operation
• Save on storage allocation
• No need to uncompress before Import
• Implemented with COMPRESSION attribute, Values supported are ALL, DATA_ONLY, METADATA_ONLY . 

3.) Data guard Compression : It includes the following features : 

• Redo is compressed as it is transmitted over a network .
• Helps efficiently utilize network bandwidth when data guard  is across data centers
• Faster re-synchronization of Data guard  during gap resolution.
• Recommended for low network bandwidth .
• Implemented with attribute “COMPRESSION”  of initialization parameter log_archive_dest_n

.4.) RMAN Backup Compression  : It compresses the RMAN backups.The followinf features are

• Supports compression of backups using "ZLIB"  algorithm .
• Faster compression and low CPU utilization compared to default BZIP2 (10g) .
• Low compression ratio  compared to BZIP2 .
• Implement  with  CONFIGURE COMPRESSION ALGORITHM  ‘value’ command where value can be High , Medium(ZLIB) and Low(LZO) .

The Oracle Database 11g Advanced Compression option introduces a comprehensive set of compression capabilities to help customers maximize resource utilization and reduce costs. It allows IT administrators to significantly reduce their overall database storage footprint by enabling compression for all types of data – be it relational (table), unstructured (file), or backup data . Although storage cost savings are often seen as the most tangible benefit of compression, innovative technologies included in the Advanced Compression Option are designed to reduce resource requirements and technology costs for all components of our IT infrastructure, including memory and network bandwidth .

The benefits of compression are manyfold
1.) Reduction of disk space used for storage .
2.) Reduction in I/O bandwidth requirements .
3.) Faster full table scans .
4.) Lower server memory usage.


Enjoy    J J J

Change Database Character Set using CSSCAN

CSSCAN (Database Character Set Scanner) is a SCAN tool that allows us to see the impact of a database character set change or assist us to correct an incorrect database nls_characterset setup. Data scanning identifies the amount of effort required to migrate data into the new character encoding scheme before changing the database character set. This information helps to determine the best approach for converting the database character set.

Before altering the character set of a database, check the convertibility of the data before converting. Character set conversions can cause data loss or data corruption. The Character Set Scanner utility provides the below two  features: 

1.) Convertibility check of existing data and potential issues. The Scanner checks all character data in the database including the data dictionary and tests for the effects and problems of changing the character set encoding (characterset). At the end of the scan, it generates a summary and exception report of the database scan.

2.) Csscan allows  also us to do a check if there is no data in the database that is  incorrectly stored.

The CSALTER script is part of the Database Character Set Scanner utility. The CSALTER script is the most straightforward way to migrate a character set, but it can be used only if all of the schema data is a strict subset of the new character set. Each and every character in the current character set is available in the new character set and has the same code point value in the new character set.

With the strict superset criteria in mind, only the metadata is converted to the new character set by the CSALTER script, with the following exception: the CSALTER script performs data conversion only on CLOB columns in the data dictionary and sample schemas that have been created by Oracle. CLOB columns that users have created may need to be handled separately

Note :  it's possible to run Csscan from a client, but this client needs to be the same base version as the database home.(i.e, oracle 11g server need oracle 11g client) .

To change the database character set, perform the following steps :


STEP 1 : Remove the invalid objects and purge the recyclebin , then take a  full backup of the database.

STEP 2 : Install the CSS utility if not install . We will get error css-00107 if css utility is not install .Install the CSS utility by running the csminst.sql script which is found in $ORACLE_HOME\rdbms\admin .

STEP 3 : Run the Database Character Set Scanner utility as
set the oracle_sid and run as
CCSSCAN  /AS SYSDBA  FULL=Y


STEP 4 : Run the CSALTER script.This script is in $ORACLE_HOME\rdbms\admin  folder .
i.>   shut down 
ii.>  startup restrict 
iii.> @csalter.plb
iv.> shut immediate
v.>  startup  

Note :  

i.> The CSALTER script does not perform any user data conversion. It only changes the character set metadata in the data dictionary.Thus, after the CSALTER operation, Oracle will behave as if the database was created using the new character set.

ii.> Changing the database characterset is not an easy task . It is quite tricky tasks and may face errors which need the oracle support . So,it's better to raise as SR for this task and involve the  oracle support .  

There  are no of links on meta-link .Check the below meta-links .

Note: 555823.1 – Changing US7ASCII or WE8ISO8859P1 to WE8MSWIN1252
Note: 444701.1 – Csscan output explained
Note: 745809.1 – Installing and configuring Csscan in 10g and 11g (Database Character Set Scanner)
Note: 225912.1 – Changing the Database Character Set ( NLS_CHARACTERSET ) [ID 225912.1]




Enjoy   J J J