O RMAN utiliza 2 tipos de compressão de blocos quando está realizando a criação dos backup sets:
- Unused Block Compression (suportado nos backups em disco e em backups de fita via Oracle Secure Backup)
- Null Block Compression (suportado em todos os tipos de backup)
Apesar dessa referência de compressão, nos bastidores o RMAN realiza um “block skipping”, ou seja, ao montar o backup set, os blocos em questão não são contemplados no backup. Na documentação oficial (link AQUI), vemos que o recurso de Unused Block Compression ocorre quando:
Além dos itens acima, também é considerado que o RMAN lerá aqueles blocos que atualmente são utilizados por algum segmento do banco de dados.
Já o “Null Block Compression” nada mais é do que a regra: blocos que nunca foram usados não serão contemplados no backup. Blocos que possuem ou já possuíram dados, serão contemplados.
Eu encontrei um post na internet de como simular esses recursos (link AQUI) e decidir reproduzir exatamente o mesmo processo, para ter passado pela experiência ver em funcionamento. Criando uma tablespace (com 100MB) e usuário:
[oracle@oel8 ~]$ sqlplus / as sysdba
SQL*Plus: Release 18.0.0.0.0 - Production on Fri Apr 30 04:50:57 2021
Version 18.13.0.0.0
Copyright (c) 1982, 2018, Oracle. All rights reserved.
Connected to:
Oracle Database 18c Enterprise Edition Release 18.0.0.0.0 - Production
Version 18.13.0.0.0
SQL> CREATE TABLESPACE ASWAN;
Tablespace created.
SQL> CREATE USER LUXOR IDENTIFIED BY LUXOR DEFAULT TABLESPACE ASWAN;
User created.
SQL> ALTER USER LUXOR QUOTA UNLIMITED ON ASWAN;
User altered.
SQL> GRANT CREATE SESSION TO LUXOR;
Grant succeeded.
SQL> GRANT DBA TO LUXOR;
Grant succeeded.
SQL> col file_name for a50
SQL> select file_name, bytes/1024/1024 from dba_data_files where tablespace_name = 'ASWAN';
FILE_NAME BYTES/1024/1024
-------------------------------------------------- ---------------
/oracle/dados/RMANDB/datafile/o1_mf_aswan_j8qfq313 100
_.dbf
Realizando um backup dessa tablespace, que até o momento, está vazia. O que podemos perceber? Apesar da tablespace ter 100MB, o backup tem pouco mais de 1MB. Ou seja, o RMAN usufruiu do recurso de Null Block Compression:
[oracle@oel8 ~]$ rman target /
Recovery Manager: Release 18.0.0.0.0 - Production on Fri Apr 30 04:53:03 2021
Version 18.13.0.0.0
Copyright (c) 1982, 2018, Oracle and/or its affiliates. All rights reserved.
connected to target database: RMANDB (DBID=3825250984)
RMAN> BACKUP TABLESPACE ASWAN;
Starting backup at 2021-04-30:04:53:19
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=89 device type=DISK
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00002 name=/oracle/dados/RMANDB/datafile/o1_mf_aswan_j8qfq313_.dbf
channel ORA_DISK_1: starting piece 1 at 2021-04-30:04:53:19
channel ORA_DISK_1: finished piece 1 at 2021-04-30:04:53:20
piece handle=/oracle/fra/RMANDB/backupset/2021_04_30/o1_mf_nnndf_TAG20210430T045319_j8qfvj5g_.bkp tag=TAG20210430T045319 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 2021-04-30:04:53:20
Starting Control File and SPFILE Autobackup at 2021-04-30:04:53:20
piece handle=/oracle/fra/RMANDB/autobackup/2021_04_30/o1_mf_s_1071204801_j8qfvkm8_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 2021-04-30:04:53:22
RMAN> LIST BACKUP OF TABLESPACE ASWAN;
List of Backup Sets
===================
BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ -------------------
260 Full 1.05M DISK 00:00:01 2021-04-30:04:53:20
BP Key: 260 Status: AVAILABLE Compressed: NO Tag: TAG20210430T045319
Piece Name: /oracle/fra/RMANDB/backupset/2021_04_30/o1_mf_nnndf_TAG20210430T045319_j8qfvj5g_.bkp
List of Datafiles in backup set 260
File LV Type Ckp SCN Ckp Time Abs Fuz SCN Sparse Name
---- -- ---- ---------- ------------------- ----------- ------ ----
2 Full 2568238 2021-04-30:04:53:19 NO /oracle/dados/RMANDB/datafile/o1_mf_aswan_j8qfq313_.dbf
Nesta etapa vamos criar alguns segmentos dentro da tablespace, que atingirá um tamanho de 88MB:
[oracle@oel8 ~]$ sqlplus LUXOR/LUXOR
SQL*Plus: Release 18.0.0.0.0 - Production on Fri Apr 30 04:58:56 2021
Version 18.13.0.0.0
Copyright (c) 1982, 2018, Oracle. All rights reserved.
Last Successful login time: Fri Apr 30 2021 04:58:07 -03:00
Connected to:
Oracle Database 18c Enterprise Edition Release 18.0.0.0.0 - Production
Version 18.13.0.0.0
SQL> CREATE TABLE TESTE AS SELECT * FROM DBA_OBJECTS;
Table created.
SQL> INSERT INTO TESTE SELECT * FROM TESTE;
73407 rows created.
SQL> INSERT INTO TESTE SELECT * FROM TESTE;
146814 rows created.
SQL> INSERT INTO TESTE SELECT * FROM TESTE;
293628 rows created.
SQL> COMMIT;
Commit complete.
SQL> col segment_name for a30
SQL> select segment_name, bytes/1024/1024 from user_segments where tablespace_name = 'ASWAN';
SEGMENT_NAME BYTES/1024/1024
------------------------------ ---------------
TESTE 88
Realizando backup da tablespace (eu já deletei o backupset anterior para não termos uma visão poluída). Agora podemos notar que devido sua utilização por parte de segmentos de banco de dados, o tamanho do backup é o mesmo do tamanho da tablespace:
[oracle@oel8 ~]$ rman target /
Recovery Manager: Release 18.0.0.0.0 - Production on Fri Apr 30 05:03:41 2021
Version 18.13.0.0.0
Copyright (c) 1982, 2018, Oracle and/or its affiliates. All rights reserved.
connected to target database: RMANDB (DBID=3825250984)
RMAN> BACKUP TABLESPACE ASWAN;
Starting backup at 2021-04-30:05:03:50
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=93 device type=DISK
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00002 name=/oracle/dados/RMANDB/datafile/o1_mf_aswan_j8qfq313_.dbf
channel ORA_DISK_1: starting piece 1 at 2021-04-30:05:03:51
channel ORA_DISK_1: finished piece 1 at 2021-04-30:05:03:54
piece handle=/oracle/fra/RMANDB/backupset/2021_04_30/o1_mf_nnndf_TAG20210430T050351_j8qgh805_.bkp tag=TAG20210430T050351 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
Finished backup at 2021-04-30:05:03:54
Starting Control File and SPFILE Autobackup at 2021-04-30:05:03:54
piece handle=/oracle/fra/RMANDB/autobackup/2021_04_30/o1_mf_s_1071205434_j8qghc1c_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 2021-04-30:05:03:55
RMAN> LIST BACKUP OF TABLESPACE ASWAN;
List of Backup Sets
===================
BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ -------------------
262 Full 89.05M DISK 00:00:01 2021-04-30:05:03:52
BP Key: 262 Status: AVAILABLE Compressed: NO Tag: TAG20210430T050351
Piece Name: /oracle/fra/RMANDB/backupset/2021_04_30/o1_mf_nnndf_TAG20210430T050351_j8qgh805_.bkp
List of Datafiles in backup set 262
File LV Type Ckp SCN Ckp Time Abs Fuz SCN Sparse Name
---- -- ---- ---------- ------------------- ----------- ------ ----
2 Full 2571030 2021-04-30:05:03:51 NO /oracle/dados/RMANDB/datafile/o1_mf_aswan_j8qfq313_.dbf
Dropando o segmento e fazendo o purge:
SQL> DROP TABLE LUXOR.TESTE;
Table dropped.
SQL> PURGE TABLE LUXOR.TESTE;
Table purged.
Com o exemplo abaixo, vemos implementado o recurso de Unused Block Compression:
RMAN> BACKUP TABLESPACE ASWAN;
Starting backup at 2021-04-30:05:09:13
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00002 name=/oracle/dados/RMANDB/datafile/o1_mf_aswan_j8qfq313_.dbf
channel ORA_DISK_1: starting piece 1 at 2021-04-30:05:09:14
channel ORA_DISK_1: finished piece 1 at 2021-04-30:05:09:15
piece handle=/oracle/fra/RMANDB/backupset/2021_04_30/o1_mf_nnndf_TAG20210430T050914_j8qgsb4n_.bkp tag=TAG20210430T050914 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 2021-04-30:05:09:15
Starting Control File and SPFILE Autobackup at 2021-04-30:05:09:15
piece handle=/oracle/fra/RMANDB/autobackup/2021_04_30/o1_mf_s_1071205755_j8qgscnl_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 2021-04-30:05:09:16
RMAN> LIST BACKUP OF TABLESPACE ASWAN;
List of Backup Sets
===================
BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ -------------------
264 Full 1.05M DISK 00:00:00 2021-04-30:05:09:14
BP Key: 264 Status: AVAILABLE Compressed: NO Tag: TAG20210430T050914
Piece Name: /oracle/fra/RMANDB/backupset/2021_04_30/o1_mf_nnndf_TAG20210430T050914_j8qgsb4n_.bkp
List of Datafiles in backup set 264
File LV Type Ckp SCN Ckp Time Abs Fuz SCN Sparse Name
---- -- ---- ---------- ------------------- ----------- ------ ----
2 Full 2571325 2021-04-30:05:09:14 NO /oracle/dados/RMANDB/datafile/o1_mf_aswan_j8qfq313_.dbf
Limpando ambiente:
SQL> DROP USER LUXOR;
User dropped.
SQL> DROP TABLESPACE ASWAN INCLUDING CONTENTS AND DATAFILES;
Tablespace dropped.
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