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12 July, 2015

RMAN -- 5 : Useful KEYWORDs and SubClauses

I begin a subseries on useful KEYWORDs and SubClauses in the RMAN Command Set.

This is on the less-commonly used KEYWORDs  (Thus, I don't show CONFIGURE CONTROLFILE AUTOBACKUP ON in this post or SET UNTIL in the next post).

(Note that specifications defined with CONFIGURE become "persistent" but can be overridden in the specific BACKUP or RESTORE runs)



COMPRESSED BACKUPSET
By default BackupSets are non-Compressed.  However, COMPRESSED BACKUPSET can be specified in either the BACKUP command OR in the CONFIGURE command.
RMAN> configure device type disk parallelism 2 backup type to compressed backupset;

old RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK PARALLELISM 4 BACKUP TYPE TO BACKUPSET;
new RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK PARALLELISM 2 BACKUP TYPE TO COMPRESSED BACKUPSET;
new RMAN configuration parameters are successfully stored
released channel: ORA_DISK_1
released channel: ORA_DISK_2
released channel: ORA_DISK_3
released channel: ORA_DISK_4

RMAN>    

RMAN> backup tablespace hemant;

Starting backup at 12-JUL-15
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=49 device type=DISK
allocated channel: ORA_DISK_2
channel ORA_DISK_2: SID=29 device type=DISK
channel ORA_DISK_1: starting compressed full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00006 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4vt_.dbf
input datafile file number=00008 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4x0_.dbf
input datafile file number=00011 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4x5_.dbf
channel ORA_DISK_1: starting piece 1 at 12-JUL-15
channel ORA_DISK_2: starting compressed full datafile backup set
channel ORA_DISK_2: specifying datafile(s) in backup set
input datafile file number=00007 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst90jct_.dbf
input datafile file number=00009 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst90jf1_.dbf
channel ORA_DISK_2: starting piece 1 at 12-JUL-15
channel ORA_DISK_2: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T182836_bt4jbnxh_.bkp tag=TAG20150712T182836 comment=NONE
channel ORA_DISK_2: backup set complete, elapsed time: 00:00:08
channel ORA_DISK_1: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T182836_bt4jbnx1_.bkp tag=TAG20150712T182836 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:16
Finished backup at 12-JUL-15

Starting Control File and SPFILE Autobackup at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/autobackup/2015_07_12/o1_mf_s_884888932_bt4jc47o_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 12-JUL-15

RMAN> 
Note how I did not have to specify "AS COMPRESSED BACKUPSET" in the BACKUP command. The CONFIGURE command had configured the DISK device type backups to be COMPRESSED BACKUPSETs as "the default".  I can override the *configured* COMPRESSED BACKUPSET by including the "AS BACKUPSET" Keywords in the BACKUP command. (Out-of-the-box, RMAN is NOT configured for COMPRESSED BACKUPSET !)



PARALLELISM in the deviceConf SubClause of the CONFIGURE command
This allows RMAN to automatically create multiple Channels when running a Backup.  Therefore, individual CONFIGURE CHANNEL commands are not required.  Note : each Channel creates a separate BackupSet !

With PARALLELISM 2, we see 2 Channels ORA_DISK_1 and ORA_DISK_2 created and used :
RMAN> configure device type disk parallelism 2 backup type to backupset;

new RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK PARALLELISM 2 BACKUP TYPE TO BACKUPSET;
new RMAN configuration parameters are successfully stored
released channel: ORA_DISK_1

RMAN>   
RMAN> backup tablespace HEMANT;

Starting backup at 12-JUL-15
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=49 device type=DISK
allocated channel: ORA_DISK_2
channel ORA_DISK_2: SID=29 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=00006 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4vt_.dbf
input datafile file number=00008 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4x0_.dbf
input datafile file number=00011 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4x5_.dbf
channel ORA_DISK_1: starting piece 1 at 12-JUL-15
channel ORA_DISK_2: starting full datafile backup set
channel ORA_DISK_2: specifying datafile(s) in backup set
input datafile file number=00007 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst90jct_.dbf
input datafile file number=00009 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst90jf1_.dbf
channel ORA_DISK_2: starting piece 1 at 12-JUL-15
channel ORA_DISK_2: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T182024_bt4hv96z_.bkp tag=TAG20150712T182024 comment=NONE
channel ORA_DISK_2: backup set complete, elapsed time: 00:00:36
channel ORA_DISK_1: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T182024_bt4hv973_.bkp tag=TAG20150712T182024 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:36
Finished backup at 12-JUL-15

Starting Control File and SPFILE Autobackup at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/autobackup/2015_07_12/o1_mf_s_884888462_bt4hwn9c_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 12-JUL-15

RMAN> 

With PARALLELISM 4, we see 4 Channels ORA_DISK_1 to ORA_DISK_4 being created :
RMAN> configure device type disk parallelism 4 backup type to backupset;

old RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK PARALLELISM 2 BACKUP TYPE TO BACKUPSET;
new RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK PARALLELISM 4 BACKUP TYPE TO BACKUPSET;
new RMAN configuration parameters are successfully stored
released channel: ORA_DISK_1
released channel: ORA_DISK_2

RMAN>      
RMAN> backup tablespace HEMANT;

Starting backup at 12-JUL-15
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=49 device type=DISK
allocated channel: ORA_DISK_2
channel ORA_DISK_2: SID=29 device type=DISK
allocated channel: ORA_DISK_3
channel ORA_DISK_3: SID=32 device type=DISK
allocated channel: ORA_DISK_4
channel ORA_DISK_4: SID=28 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=00006 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4vt_.dbf
input datafile file number=00009 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst90jf1_.dbf
channel ORA_DISK_1: starting piece 1 at 12-JUL-15
channel ORA_DISK_2: starting full datafile backup set
channel ORA_DISK_2: specifying datafile(s) in backup set
input datafile file number=00007 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst90jct_.dbf
input datafile file number=00011 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4x5_.dbf
channel ORA_DISK_2: starting piece 1 at 12-JUL-15
channel ORA_DISK_3: starting full datafile backup set
channel ORA_DISK_3: specifying datafile(s) in backup set
input datafile file number=00008 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4x0_.dbf
channel ORA_DISK_3: starting piece 1 at 12-JUL-15
channel ORA_DISK_1: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T182301_bt4j05dl_.bkp tag=TAG20150712T182301 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
channel ORA_DISK_2: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T182301_bt4j05f7_.bkp tag=TAG20150712T182301 comment=NONE
channel ORA_DISK_2: backup set complete, elapsed time: 00:00:03
channel ORA_DISK_3: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T182301_bt4j05jz_.bkp tag=TAG20150712T182301 comment=NONE
channel ORA_DISK_3: backup set complete, elapsed time: 00:00:03
Finished backup at 12-JUL-15

Starting Control File and SPFILE Autobackup at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/autobackup/2015_07_12/o1_mf_s_884888584_bt4j08mc_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 12-JUL-15

RMAN> 

In the latter case, although 4 Channels were created, RMAN actually used only 3 Channels.  So, RMAN can still automatically adjust the allocation of datafiles across the Channels.



MAXSETSIZE in the BACKUP command.
A BACKUP run (whether at Database level or for specific Tablespace(s) or Datafiles or ArchiveLogs), by default, creates BackupSets.  However, a BACKUP run can create multiple BackupSets.
This can happen when :
a.  The number of datafiles (or ArchiveLogs) in the Backup is "large"  (e.g. by default, unless FILESPERSET is specified, a max of 64 datafiles make up one BackupSet).
b.  Multiple Channels are used for the BACKUP run
c.  The PARALLEL Keyword is used  (as demonstrated above)
The MAXSETSIZE is actually a way to specify the maximum size of a BackupSet.  A datafile cannot span BackupSets, so the MAXSETSIZE must be, at least, as large as the largest datafile in the database.

SQL> select max(bytes)/1048576 from dba_data_files;

MAX(BYTES)/1048576
------------------
              1259

SQL>   

RMAN> backup as compressed backupset database maxsetsize 1200M;

Starting backup at 12-JUL-15
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=49 device type=DISK
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of backup command at 07/12/2015 18:14:37
RMAN-06183: datafile or datafile copy /home/oracle/app/oracle/oradata/orcl/sysaux01.dbf (file number 2) larger than MAXSETSIZE

RMAN> 

Thus, we can see that Oracle automatically checks the largest datafile against the MAXSETSIZE. Note, also, that it does not estimate a Compressed size for this check, even though I've specified "AS COMPRESSED BACKUPSET".



MAXPIECESIZE
Just as a Backup can consist of multiple BackupSets, a BackupSet can consist of multiple BackupPieces.  A BackupSet can consist of multiple datafiles and a datafile may be split across multiple BackupPieces. However, unlike the MAXSETSIZE that can be specified with the BACKUP command, MAXPIECESIZE is specified with the CONFIGURE CHANNEL. (However, unlike the MAXSETSIZE which runs a check before beginning the backup, this limit is applied when creating backuppieces !)

RMAN> configure device type disk parallelism 1 backup type to compressed backupset;

old RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK PARALLELISM 2 BACKUP TYPE TO COMPRESSED BACKUPSET;
new RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK PARALLELISM 1 BACKUP TYPE TO COMPRESSED BACKUPSET;
new RMAN configuration parameters are successfully stored
released channel: ORA_DISK_1
released channel: ORA_DISK_2

RMAN> configure channel device type disk maxpiecesize 25M;

new RMAN configuration parameters:
CONFIGURE CHANNEL DEVICE TYPE DISK MAXPIECESIZE 25 M;
new RMAN configuration parameters are successfully stored


RMAN> configure channel device type disk maxpiecesize 5M;

old RMAN configuration parameters:
CONFIGURE CHANNEL DEVICE TYPE DISK MAXPIECESIZE 25 M;
new RMAN configuration parameters:
CONFIGURE CHANNEL DEVICE TYPE DISK MAXPIECESIZE 5 M;
new RMAN configuration parameters are successfully stored
released channel: ORA_DISK_1

RMAN> backup datafile 6;

Starting backup at 12-JUL-15
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=49 device type=DISK
channel ORA_DISK_1: starting compressed full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00006 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4vt_.dbf
channel ORA_DISK_1: starting piece 1 at 12-JUL-15
channel ORA_DISK_1: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T184704_bt4kf8jm_.bkp tag=TAG20150712T184704 comment=NONE
channel ORA_DISK_1: starting piece 2 at 12-JUL-15
channel ORA_DISK_1: finished piece 2 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T184704_bt4kf9jn_.bkp tag=TAG20150712T184704 comment=NONE
channel ORA_DISK_1: starting piece 3 at 12-JUL-15
channel ORA_DISK_1: finished piece 3 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T184704_bt4kfbjs_.bkp tag=TAG20150712T184704 comment=NONE
channel ORA_DISK_1: starting piece 4 at 12-JUL-15
channel ORA_DISK_1: finished piece 4 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T184704_bt4kfcjy_.bkp tag=TAG20150712T184704 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:04
Finished backup at 12-JUL-15

Starting Control File and SPFILE Autobackup at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/autobackup/2015_07_12/o1_mf_s_884890028_bt4kfdn9_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 12-JUL-15

RMAN> 

SQL> !du -sh /NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T184704*
3.1M    /NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T184704_bt4kf8jm_.bkp
3.1M    /NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T184704_bt4kf9jn_.bkp
3.1M    /NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T184704_bt4kfbjs_.bkp
68K     /NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T184704_bt4kfcjy_.bkp

SQL> 

The configured MAXPIECESIZE limit applied to the compressed piecesize. !



FILESPERSET
I have mentioned FILESPERSET earlier in this post and in another blog post.  This Keyword allows us to limit a BackupSet to a certain number of files.

RMAN> backup as compressed backupset tablespace HEMANT filesperset=6;

Starting backup at 12-JUL-15
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=42 device type=DISK
channel ORA_DISK_1: starting compressed full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00006 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4vt_.dbf
input datafile file number=00007 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst90jct_.dbf
input datafile file number=00008 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4x0_.dbf
input datafile file number=00009 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst90jf1_.dbf
input datafile file number=00011 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4x5_.dbf
channel ORA_DISK_1: starting piece 1 at 12-JUL-15
channel ORA_DISK_1: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T222911_bt4yfrd6_.bkp tag=TAG20150712T222911 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:45
Finished backup at 12-JUL-15

Starting Control File and SPFILE Autobackup at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/autobackup/2015_07_12/o1_mf_s_884903397_bt4yh670_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 12-JUL-15

RMAN> 

With FILEPERSET=6, all 5 datafiles are in one BackupSet (as long as the total size is less than the limit on a BackupSet Size / MAXSETSIZE).

Now, let's rerun the backup with FILESPERSET=1

RMAN> backup as compressed backupset tablespace HEMANT filesperset=1;

Starting backup at 12-JUL-15
using channel ORA_DISK_1
channel ORA_DISK_1: starting compressed full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00006 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4vt_.dbf
channel ORA_DISK_1: starting piece 1 at 12-JUL-15
channel ORA_DISK_1: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T223210_bt4ymbh5_.bkp tag=TAG20150712T223210 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
channel ORA_DISK_1: starting compressed full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00007 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst90jct_.dbf
channel ORA_DISK_1: starting piece 1 at 12-JUL-15
channel ORA_DISK_1: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T223210_bt4ymfnl_.bkp tag=TAG20150712T223210 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
channel ORA_DISK_1: starting compressed full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00008 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4x0_.dbf
channel ORA_DISK_1: starting piece 1 at 12-JUL-15
channel ORA_DISK_1: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T223210_bt4ymjq8_.bkp tag=TAG20150712T223210 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
channel ORA_DISK_1: starting compressed full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00009 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst90jf1_.dbf
channel ORA_DISK_1: starting piece 1 at 12-JUL-15
channel ORA_DISK_1: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T223210_bt4ymmt5_.bkp tag=TAG20150712T223210 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
channel ORA_DISK_1: starting compressed full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00011 name=/home/oracle/app/oracle/oradata/HEMANTDB/datafile/o1_mf_hemant_bst9o4x5_.dbf
channel ORA_DISK_1: starting piece 1 at 12-JUL-15
channel ORA_DISK_1: finished piece 1 at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/backupset/2015_07_12/o1_mf_nnndf_TAG20150712T223210_bt4ympxr_.bkp tag=TAG20150712T223210 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
Finished backup at 12-JUL-15

Starting Control File and SPFILE Autobackup at 12-JUL-15
piece handle=/NEW_FS/oracle/FRA/HEMANTDB/autobackup/2015_07_12/o1_mf_s_884903546_bt4ymtdr_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 12-JUL-15

RMAN> 
Now, I can have each datafile go into a separate BackupSet. The advantage of using FILESPERSET in one context was shown in a previous post. Another advantage is obvious when we have to restore a single datafile. If we had 16 datafiles in a BackupSet, and we need to restore a single datafile, RMAN still has to read the entire BackupSet of 16 datafiles to be able to restore that single datafile.  If each BackupSet was limited in size by using MAXSETSIZE and/or FILESPERSET, restoring a single datafile can be faster as the Restore has to read a smaller BackupSet.

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