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Does RMAN backup unused blocks as well ?

flying_penguinFeb 18 2016 — edited Feb 28 2016

DB version :

OS         : RHEL 6.6

If I remember correctly, RMAN backs up only used blocks. I created a 20GB tablespace as shown below. When I backed it up, the backup piece is only 4MB in size as shown below.

SQL> create tablespace TEST_TS3 datafile '+DATA_DG' size 20G

autoextend on

maxsize 32767m

extent management local

uniform size 4M

segment space management auto; 

Tablespace created.


SQL> select tablespace_name, bytes/1024/1024 from dba_Data_Files where tablespace_name = 'TEST_TS3';

TABLESPACE_NAME                BYTES/1024/1024

------------------------------ ---------------

TEST_TS3                                 20480

Backing up TEST_TS3 tablespace which is not used by any segments ( empty )

RMAN> backup tablespace TEST_TS3 format='/u05/RMAN_BKP/TBS_%U_%T.rmbk' tag=TEST_TS3_BKP ;

Starting backup at 18-FEB-16

using target database control file instead of recovery catalog

allocated channel: ORA_DISK_1

channel ORA_DISK_1: SID=4941 instance=HMAGPRD2 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=00144 name=+DATA_DG/hmagprd2/datafile/test_ts3.413.904152141

channel ORA_DISK_1: starting piece 1 at 18-FEB-16

channel ORA_DISK_1: finished piece 1 at 18-FEB-16

piece handle=/u05/RMAN_BKP/TBS_03qu8hg2_1_1_20160218.rmbk tag=TEST_TS3_BKP comment=NONE

channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01

Finished backup at 18-FEB-16


$ ls -lh /u05/RMAN_BKP/TBS_03qu8hg2_1_1_20160218.rmbk

-rw-r----- 1 oracle oinstall 4.1M Feb 18 17:29 /u05/RMAN_BKP/TBS_03qu8hg2_1_1_20160218.rmbk

But , in real production world, RMAN backup size seems to be much higher.

The following production DB is 27.5 TeraBytes in size as reflected in the physical layer ( DBA_DATA_FILES.BYTES column )

SQL> select sum(bytes/1024/1024/1024) bytesGB from dba_Data_Files;




But, at the logical layer , the space consumed is only 16.5 TB as reflected in DBA_EXTENTS.BYTES column

SQL> select sum(bytes/1024/1024/1024) bytesGB from dba_extents;




But, the RMAN Backup size came to 20 Terabytes which is nearly 4TB more than the space consumed at the logical layer (DBA_EXTENTS.BYTES).

Why is this ?

This post has been answered by Vsevolod Afanassiev on Feb 18 2016
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Locked on Mar 27 2016
Added on Feb 18 2016